The temperature differential between the beer and ambient temperatures is consistent - even at low temperature fermentation. Here's the data from a Helles Lager brewed at 10.5C.
It seems that the yeast generate exothermic activity (heat) based on the amount of sugars in the wort. I guess we all started brewing without using a sophisticated fermentation chamber. Just imagine what the yeast will do to the beer temperature if left to their own devices!