On an 80wh battery, say you go from 7hrs to 8hrs, so- 10wh saved over 8hrs. Thats a 1.125watt difference.

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.