Charging efficiency. Wall to wheels is a 12-17% (6.6 to 4.8 KW) deduction depending on charging speed.
I don't see temps anywhere near yours and I reset my trip meter every morning but still have a lot of low 3's. defrost, heat, speed, whatever.
Agreed that charging losses need to be accounted for, and should be, in the cost per mile for comparison to ICE. But as for comparison between summer and winter, I am really using solely the car's readout of M/Kwh to summarize how much fewer miles I go in the iwnter with heat etc. Speed is one factor we seldom need to account for- I remember doing a 0 to 100 MPH a year or so ago just for S and G, since then I don't think I've seen the plus side of 60 more than a mile or 2. PS the 0-100 took a few seconds but was still a thrill- the Plus has gobs of power for my needs.