Even in summer when the battery is at optimum temperature without any electrical heating that is good going.
Not sure which generation of BMW electric drive train the Mini has but it must be a very recent one (perhaps 4th or 5th generation) to be able to turn in figures this good. BMW have been steadly improving the efficiency of their electric drive sub-assembly components using the i3 as a test bed. I have never got my 2016 i3 better than 5.5 miles per kWh. The Leaf before that would often achieve 5.5. mile per kWh in summer. Hoping for better when I switch to a bigger battery i3 without a REx.
Bjorn on youtube was equally impressed and confused how it was this good. It does not look particularly aerodynamic either. BMW clearly have some secret sauce and perhaps this is why they are claiming their 5 series EV will have this 400+ mile range.
I get 10mi/kWh in the Zoe one way, 5mi/kWh back. (Average of those two is 6.6mi/kWh, of course! )
It is quite easy to get stunning short-commute rates if the overall journey is both short urban and negative elevation because the change of elevation will dominate compared to relatively low energy demands in the urban route.
Not saying that you aren't doing well, but need to see the change of elevation and likewise figure on the return route to make sense of it.