FPS can be finicky. It's really an average over time. Depending on how long you play the game for and other conditions (what's running, etc), a difference of +/- 2 fps is nothing to really be concerned about.
In fact, when you do a bench mark, you really should disable as many services (network, windows update, anti virus, etc) as possible and then, you run your benchmark at LEAST 3 times to get an average.
Have you tried running the game again later on (without rebooting) to see if it dropped another 2 or if it went back to the original or even increased? I really think you're grasping at straws here and are worrying over nothing.