I think you may be misunderstanding how the battery display works, basically the 310 miles that it shows when the car is brand new (and for some thousands of miles afterwards) it the best shape the battery will be in, which is rated at approximately 240 Wh/mi.
So essentially if you notice the number (when 100% charged) being below 310 miles, that means your battery has irreversibly lost a tiny bit of range regardless of how efficiently you drive, since it is still rated at the 240 Wh/mi mark.
Now of course through efficient driving you could still get MORE than the 310 miles (or whatever your new maximum range is) but again that would be at a Wh/mi rate different than the default 240 Wh/mi that the battery is based on.
tl:dr whatever number the battery shows at full charge is the amount of miles you can get if you drive at 240 Wh/mi, thus if we see a drop in the number of miles you get on a full charge, you have permanently lost range due to slight battery degradation.