Change in efficiency calculation over time.

Last October I had 176Wh/km and that was calculated as 77% efficiency.

This October I have 177Wh/km and that is calculated as 83% efficiency.

Has the efficiency multiplier changed over time and if so when did it change and how/for what reason.

Or is this due to some rounding effect/bug where all the individual trip efficiencies average differently than the Wh/km values.