Well, Apple have explained what's going on, saying that the battery life indicator is showing a higher percentage than the battery actually has. Don't worry though - there's a quick fix you can do, and we're going to tell you how to do it.
Ready? First off, restart your iPhone. Then, when it is back on, go to Settings, then General, and then go to Date & Time. Make sure that Set Automatically is turned on. And that should be it. If you want to make extra sure, then restart your iPhone twice, so it can detect the right timezone, and it should be fine after that.
Seems like a nonsense that you should have to do that, but there you go. It seems to fix the problem, which is software based according to Apple. Quite why the battery indicator and timezone of your phone are connected, is another matter.
Either way, this came to everyone's attention after a number of iPhone owners started complaining about it online, saying that their iPhone 6s and 6s Plus models were running out of juice much faster than the battery indicator was showing. At least this isn't a problem with the battery itself, as that would be a pain to sort out and live with.
Apple haven't found a permanent solution yet, but we can assume they're working on it, now they've acknowledged the problem and say they're aware of the complaints. Until then, follow the instructions above.