Yeah, there are two batteries, the one in the earbuds and the one in the container. There's no way in BLE to transmit both values - and choosing either one is lying to the user about something.
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.
See my other replies in this thread — it’s totally possible to do with standard Bluetooth, yet Apple doesn’t do it. So your “fairly sure” assumption that Apple would make use of this feature if it existed seems to be wrong.
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.