Battery fully charged at 96 percent?

My computer recently did something strange tonight. I was charging it up like normal. When I suddenly noticed the green charger light was on, and it said 100 percent. However, after unplugging it I noticed it was only at 96 percent. I plugged it in again, and it still said it was fully charged. The only thing I did different tonight as that I started charging it before the battery meter turned red. I got nervous when I noticed it was draining slowly so that's why I plugged it in too soon. It was still low enough though.

I'm just wondering if this is normal or something to be concerned about? My machine is a later 2011 model and the battery is still fairly new. I got it during the holidays.