It's because a device located next to a radiator and bolted to a hot water pipe is not well placed to measure an absolute temperature of the room. What you therefore need is just a relative measurement (hence the 1 to 6) and you can adjust it up and down until the desired comfort level in the room is maintained.
Think of it like the volume control on your TV or hifi - we cope quite happily despite most not being measured in decibels.
I think more due to the large slewing range, a mechanical TRV has a slewing range of around 3°C, i.e. difference between fully open and fully closed, the position may not be prefect, you want to measure the return air, and the radiator causes a thermal circulation, simple like this
however in real terms items like windows can change it a bit
this is why as a compromise the wall thermostat was placed on the wall facing the radiator being in the thermal flow but as far from the radiator as practical, the Myson fan assisted radiator worked very well due to the forced circulation, however it is not always good to have whole room at the same temperature, some times it is better to allow pockets of warm or cool air, either where sun shines through a window, or window loses more heat than walls, I lived in a house with hot air central heating before the days of double glazing, and whole house even heat, but heat losses were high so expensive to run.
The first electronic TRV head I got,
had two sensors, one for water and one for air, and the water one compensated for the direct heat from the radiator, however the built in anti-hysteresis software was a bit OTT, so in the morning set to 22°C for one hour then down to 20°C to counter the OTT anti-hysteresis software, and once the lock shield was set except for where morning sun caused a problem with slight overheating they were spot on, however expensive, the second type I got were far cheaper,
no wifi, although did have bluetooth, so either you can adjust with phone or link to another head so if two radiators in the same room they can work together.
I got the first type as told they would work with Nest, however when Nest was bought out by Google the support was dropped, so only option now with Nest is set the same schedule. We are told the Drayton Wiser TRV heads like the Nest wall thermostat works out how long it takes to heat a room, so the problem I had with OTT anti-hysteresis software is avoided.
However all these clever algorithms depend on things like doors being always the same, in spite of the
being cheap, it actually has a window open detect to turn off the heating in the room for a settable time, should a rapid temperature drop be detected, I have the one in kitchen set like that so it switches off while we unload to car and have back door open.
We have had all sorts of cleaver ideas for thermostats, this one
used a mark/space ratio as it approached the target temperature to stop it over shooting, sounds great, and worked well with most oil boilers, however with the gas modulating boiler every time it is switched off externally when it switches on again it does so at maximum output, so the thermostat completely messed up the boilers own algorithms, and caused the boiler to run less efficiently.
So either we want the wall thermostat to turn boiler up/down, not on/off, or the wall thermostat only stops the boiler cycling in the summer, or there is a link between the wall thermostat and the TRV heads when using a gas boiler.
Much depends on the make and model of the gas boiler, it is all well and good saying the EPH thermostats can be set as master/slave so work with a number of hard wired motorised valves and OpenTherm, but if the boiler is not OpenTherm enabled that clearly will not work. Like the Mac computer which is very good, but does it's own thing and will not run third party software, so with Boilers we have Bosch which has not allowed third party thermostats to interface with it.
So there is one odd one out of the smart thermostats, designed to be simple on/off but work with the linked TRV heads, so when working with TRV heads it acts more like a hub. Drayton also seems to have a system to work with hard wired zone valves and OpenTherm, although never worked with it, but clearly a TRV is a zone valve, maybe not hard wired, but never the less it has to qualify as a zone valve, but the problem is the balance between installation cost, running cost, and how well it controls the home.
So all central heating is a compromise. If my oil costs £500 a year, and very cleaver control can reduce that to £400 a year and the system will likely last 15 years, then break even point is £1500, spend any more and you will not get the money back, specially if you include interest. So back to the compromise, maybe two or three devices to measure temperature and decide if boiler runs, placed in the rooms most likely to be under temperature or critical rooms, and other rooms using stand alone TRV heads which are much cheaper.
But the whole system has to include what is already installed, all well and good saying rip it all out and installed EvoHome, or Wiser, but then we have the £1500 limit, so it has to integrate with what is already there. No good singing the praises of OpenTherm if the boiler is not OpenTherm enabled.
The big question with hard wired zones, is would combining the zones together work better? And then use the TRV heads to set up zones? There is no one size fits all answer, likely in most homes you will need more than one thermostat that can switch heating on, be it two hard wired wall thermostats in parallel or a hard wired wall thermostat with wireless link to other thermostats, possibly a TRV head but not all the TRV heads need to be linked.
Mothers old house two wireless thermostats in parallel
one receiver above consumer unit and one below it, trying to keep some distance between the two with EMC in mind. So one thermostat in kitchen and one in hall, neither ideal positions, but as a pair they worked well.
Aim of wall thermostat:-
1) Ensure boiler is running when required.
2) Stop boiler when we get warm weather.
It is not to control room temperature, that is what the TRV is for.