It's not uncommon (at least for me) to have a low earbud battery level (because I've just binged Slow Horses) or a low container battery (because I've just charged the earbuds from the container for the third time and drained the container). There's a suggestion above that you should "just choose the lowest one because 99% of the time that's what you're interested in", except that's not true in the second case.

I'm fairly sure that if you could report both, then Apple would report both using this hypothetical standard method, but since you can't, and there's no easy way to just "choose one" without misleading the user about something, they choose to do it properly, even though that means it's an Apple-only thing.