I have another vehicle, not a clarity, but will chime in here. My EV range is always 2 miles below stated range. At night when I charge it, in a carport,(basically outdoor like) it has always been below 20F and often quite colder. One time at 0 or so I think I got 3 below stated range.
I see it as completely a function of what temp the battery is charging at. Lower temps and batteries are more reluctant to accept charges.
Functionally the battery recharges depending on application with regeneration, and with the increased temps in the day I expect it really has not much of a real difference in how far I go on EV mode when in town. My first ride to town is basically downhill, and I use zero miles. My return trip back home is uphill, and while it is a seven mile trip uphill, it uses more like nine or so Ev miles. More energy is expended going uphill. So there is variance, these are only approximations.
How I use the car seems to directly affect how much EV miles I use. So I want to extend range I simply drive a bit more conservative. I think that is way more influencing the thing than the loss of 2 miles or so due to temps. I guess they derive the numbers from flat level ground at normal temps.
If as some state here with the clarity it is averaged in use application..my driving in town in flat level at normal temps but within ten or so miles to home it is significantly uphill(or downhill) and much colder. So any approximation with such deviation in terrain and temp would be prone to inaccuracy. In the clarity or my vehicle.