The battery meter on your phone is lying to you. Why don't handset manufacturers trust us to see the real battery level? This is something that has bugged me for ages and since it's also evident on the HTC One, I thought i'd post a little rant.
So what's the deal? Basically, the battery meter display on most devices doesn't correlate correctly with the actual battery left. Typically, they seem to initially indicate more is left than really is to begin with, becoming more representative as the battery is depleted. Why? I don't really know... maybe it's to give the impression that your battery is 'doing better' than it really is?
This is an analysis of the battery images on the HTC One. The 40% indicator is the only one (aside from full and empty of course) that is actually correct and no, it's not an error - the 80% and 90% images are exactly the same! I've pulled out the 10 images and correlated them with the maximum battery percentage for which they are displayed.
If you're on a nice big screen it might not be immediately evident how significant the 'errors' are, but on device, it's really quite clear.
I'm going to reach out to HTC to see if they can explain why this is the case... any thoughts? Battery meters with a 100 image split instead of 10 are definitely the way to go (i'm not a fan of textual percentage representation myself)...