Why does my lumia 1520 battery reach 100% from 60+% in 20 minute?

[WPCentral Question]

First of all, I am using insider app to track my phone battery level. So, after since i updated my phone to lumia denim, my phone battery had been full charged in such way. It start charging as normal but as soon as the battery level reach 60+% it went to full charge in another 20 minute time. Is there anyone out there having the same issue? i found it kinda weird as the 60+% that i have mention is keep on dropping. Last time it'd been 67% or 68% or 69% but now it'd been 61% or 62%.