There could be other factors - what is your fermenter made out of? Plastic is a better insulator than stainless, so it will trap heat. What volumes are you fermenting? Surface area to volume ratios will affect heat transfer. What kind of airflow is present that might alter heat transfer? Is it in a fridge or a freezer or a room? What is the SG, what yeast are you using, and how vigorous is the fermentation?
Ultimately we know that fermentation will cause the temp to rise, and you need to monitor it to find out the difference for your setup and recipe. It sounds like 2-8F is a reasonable range for people who aren't sure.
For the me the largest effect on temp rise seems to be from the location of the carboy in the freezer.