Remarkable

So when my (rM2) tablet is low on battery, I charge it overnight. And when I unplug it in the morning, it's 90% charged. If I plug it again, then it goes up to 100%, but never by itself. It aways stops at 90. Is anyone else experiencing this? Or am I missing something? It's only few months old and it's been doing it since the beginning. Appreciate any replies!