I'm sure a lot of people can leave taps running for at least a minute waiting for hot water to come through.
Indeed, a lot of water is wasted like that - at home, there is enough cold water at the kitchen sink to fill mum's small plant watering can. But at G/Fs house, you also have to get the combi to heat up as well - so learning to turn the hot tap on and off to fire it up, give it time to warm up, and then let the hot tap run to flush the (unlagged) pipes before jumping in the shower. At home, the shower is about 2' of pipe from the hot water cylinder so is almost instant.
As an experiment, when I installed a thermal store in the flat, I measured the standing losses. You know how it is, over in the plumbing section if you mention thermal store they all go <suck> "pile of sh1te, wasteful of energy, blah, blah, ...". So my wasteful thermal store lost 80W - I tested by leaving it maintained by just the immersion heater for a week with nothing else turned of (flat was empty at the time). t the same time, my house next door was also empty, so I left everything turned off except the WB combi with it's "keep hot for instant hot water" function at it's default of on. it used the equivalent amount of gas to 160W of standing losses. Given that the thermal store also gives a far superior hot water supply, and much better heating, I know which I prefer - without the keep warm function, that "modern" boiler can take up to a whole minute before hot water comes out of it !
I'm not sure what any of this has to do with airtight electrical boxes, but hey!
It's called thread drift.
You will be drinking water that has been in (and in your case several times around) the cylinder which is 'stale'.
If you have a cylinder. Could have a heat bank or thermal store in which case there is very little stored hot water in the DHW system.
A more significant reason is that if you have an open vent cylinder, the hot water you draw has been stored in the header tank for an indeterminate time (possibly at a "bug breeding" temperature in summer). The manufacturer may have supplied a close fitting lid, but the plumber almost certainly butchered that during installation if he could be arsed fitting it at all*.
At school, a physics teacher told us how at one factory he'd worked in, the hot taps were hot enough to make hot drinks - so they did. Then they noticed that their drinks "tasted funny". On investigation, a decomposing rat was pulled from the header tank - and they stopped making drinks from the hot tap
* Speaking from observation. Generally observe either no lid at all, a badly fitting butchered one, or at best an offcut of old chipboard thrown on top (which is now saturated and starting to fall apart).