Default [Q] GT-I9500: Original battery shows 2100mah capacity.

Hi there,

I have a Galaxy S4 int. version.
Since last few days it wasn't getting enough battery back up from stock battery.
So I installed few application check battery stats.
But there I found a problem. That battery is not showing the actual capacity which is 2600mAh.
It is showing 2100mAh instead.
Can any one tell me what could be the issue? It is the original battery which came with phone.
Application which I used to check stats is Battery Monitor Widget.