And not to mention the discharge efficiency curve. For LiPo batteries you will see a sort of “coasting” when the battery is at about 3.7v if your load is sufficiently small. But when it’s at 4.0v it will seem to drain quicker.

I have a 10000mAh battery powering a device that uses 10uA every 30 minutes for about 10 seconds. On one charge it should last about 1 year maybe a little more. The first 2 months it seemed like it lost a lot more juice than was expected but since then it settled in and is now ahead of its estimate. Which means it will speed back up as it goes down below 3.7v.