Wasn't that a bug that was fixed weeks ago? Like early August? If you are not averse to this feature then it is better to simply make sure you are running the latest version.
I propose the below as various factors that can be larger:
Slower fan speed because of lower ambient temperature.
Different dark/light ratio and/or adaptive screen brightness.
Wifi spectrum congestion, variable power levels to maintain proper SNR.
Wifi/ethernet- broadcast packets.
The list goes on. Most of these are below a watt, but demonstrate the point that you've got a lot more variables than just one setting in a browser.