I think you're taking that % indicator way too seriously... there's no way to actually know the % charge of a Lith-Ion battery. It's an estimate based on voltage behavior while charging.
The only way to REALLY know precisely how much energy can be drawn from a lithium ion battery is to actually drain it to whatever your voltage cut-off is, and track what you've drawn along the way.
Everything else is an estimate. Your car is estimating the battery based on observations. Sometimes your % can actually go UP while the car sits parked. I've seen it go up 3-5%. Sometimes the % can drop more than you expect, because the battery is basically saying "yeah, what I told you earlier, that was wrong."
Anyway, the % is an estimate. The consumed wh/mile is a lot more trustworthy.