OP - i recently fitted the same unit to an old boiler (ideal mexico 2) and experienced the same behaviour.
I put a digital thermometer next to the CM927 display, and sure enough the stat was doing its job. Some days the rads barely get warm yet its sufficient to raise the temp of the room enough. What I find though, is that if the room is at 20c for a while I occasionally start to feel cold, even though the room is a constant temperature - odd! But its me I think, as the temp is constant.
this is one reason I asked the question as sometimes we felt a little cold even though the room temp was at the set temp.