It's an iOS 8.x.x bug. Every version of iOS 8 is affected, iOS 9b1 is affected. I've heard iOS 9b2 fixes this but I have not confirmed this myself so no guarantee. These are all my known battery percent skips. 61%>59%, 55%>53% (heard of it but haven't confirmed), 31%>29%, 28%>26%, and I think there's one in the teens as well, like 18%>16% or 16%>14% or something. Yeah, a lot more than you thought.