2 port valve wiring

[KevC1]

Is there a reason honeywell 2 port valves have a 6 core wire on the 28mm, and a 5 core on the 22mm.

The reason I ask is that I was converting a gravity system to a c-plan. I put a normal 22mm 2 port on it. When I came to wire it, it needed white core to turn boiler on for heating. Had to buy another 2 port valve, and swap the heads.

Is there a reason for this?

Thanks.

 

[seco services]

on a s plan when the valve closes the boiler/pump stop.
on a c plan when the valve closes the boiler only fires up
pump white powers orange through m/switch
when valve opens grey powers orange through m/switch no pump needed.