basically the CPU heats up the air in the case, that heats up room you are in, that heats up the house...
By waiting 3 hours you get it to a point where it saturates the local area, the room, etc, and gets a few degrees higher, and is kept in check by the house AC (or just the outside temperatures if it is winter).
Ofcourse, those things still fluctuate a bit. But really there isn't much point to test it.it makes more sense to check max temp 30 seconds later, and then do a long term stability test, rather then checking temperature after long time load (to see EXACTLY how much higher it went).