I know that feeling! I'm not Fitbit user, I use miBand, but it always gives me a different number of steps. Sometimes it is higher, but sometimes it's lower. Later on, I realized thatmy steps keep counted even tho I didn't walk at all, for example when I ride a motorcycle, drive a car, or take a bus. But since it's cheap and the number makes me happy (because it makes me look like an active person who can walk 10.000 steps per day), I don't mind it.

Anyway, your activity number is aesthetic. 6699. Such a lucky number.