Just b/c a device's battery meter shows "full" doesn't mean the battery is at 100%.
Exactly this. The total capacity of a Li battery will naturally deplete over time. The battery may still charge to “100%”, but it’s only charging to 100% of its exisiting capacity, not necessarily the rated capacity when it was new. For example, the Equinox battery is rated at 5000 mAh from the factory - a brand new, fully charged Equinox battery should be somewhere close to that. After a couple years, it may still charge to 100% and show full on the indicator, but the battery may only be capable of holding, say, 4100mAh depending how the battery was treated by the user and other variables…the reduced capacity manifests itself as a shorter total run time despite starting at “100%”. Eventually, the battery capacity depletes to the point that it can no longer deliver the required voltage for any amount of time. An example is my wife’s “ancient” iPhone 8, which recently died…it would charge to 100%, and do it fairly quickly - then as soon as it was unplugged, you could watch the battery percentage fall and phone would shut down within 10 or 15 minutes.
Here’s another example: I fly drones for my side business, and the pic below shows the battery status page for one of the Li-Po batteries that’s currently over 6 years old for one of my older drones. The original maximum capacity for the battery was 4900 mAh. As you can see, the current maximum capacity after 6 years of use is only 3816 mAh (3816 divided by the original 4900 equals 78%, the “battery life” shown in the display). Instead of the original 22 minutes of flight time I used to get, this battery only delivers about 12 minutes of flight time under ideal conditions, so I’ve retired it from service. But my point is, the battery still charges to “100%”…in the pic, the battery is charged to 3806 mAh of it’s 3816 mAh present capacity, and the battery meter shows 100% at the start of a flight. By way of comparison, the second pic shows a slightly newer example of the same battery type…that one is maybe 4 or 5 years old, but it’s maximum capacity is still pretty much at factory specs.