MacBook Pro battery level drops while plugged in.

I need some help with the recent issue that I have been noticing on my MacBook Pro. Recently, I have been noticing that my MacBook Pro battery levels sometimes drop by 1% point after a few hours of use in OS X while the computer was plugged in with the MagSafe adapter. Recently, when I installed Windows 7 on my MacBook Pro I remember that before installing Windows 7 my MacBook Pro had a 100% charge on it and the computer was connected to a power source. Upon finishing the Windows 7 installation to my surprise I found that my battery level had dropped to 81%. The Windows 7 task bar indicated that my computer power was plugged in but not charging, I quickly solved the issue by unplugging the MagSafe adapter and then repuggling it which re engaged the battery charging process. Yesterday, after using my computer on battery for a small period in my Speech class of time and then re-charging which again charged the battery fully. I kept it connected to the MagSafe adapter for the next hour. After that for a moment, I went to Windows 7 in Bootcamp to run a Flight Simulator for about 5 minutes to show a friend one of the aircraft I downloaded. Since the MagSafe light was green I did not glanced to the battery indicator, I then proceeded to shut down the computer and left to go home. When I got home and booted my Mac, to my surprise, I found that the battery was at a 93% charge, so then again I plugged in my computer and proceeded to charge it. At this time, I also decided to run the Apple Hardware Test 2 times to see if there was any indication of a problem with my computer, however both passes gave a clean bill of health.

After this, the only thing I thought to do was to reset the SMC and see if that could fix my problem. So far, I haven't seen the problem present itself.

Therefore, I was is it normal that under some circumstances for the battery charge levels to drop while the computer is plugged in? I had never experienced something like this before either on OS X and Windows 7, nor when running the Flight Simulator on Windows 7 with the adapter plugged in (has been that way for the last year).

Thanks for any assistance on this matter.