You're supposed to calibrate your stride to a normal, every day walking stride, and then your Fitbit is supposed to adjust that when it detects steps that vary from a normal, every day walking intensity. So, if you're shuffling it is supposed to shorten the step and lengthen it when you're power walking. But, I'm not sure it always increases it enough and things like hills can confuse it further. I get an awful lot of my steps from walking for exercise, so I've calibrated it to do that fairly well, even if it may then over-estimate my distance from normal walking. I do it by using MapMyWalk to get the distance and my Fitbit to get the number of steps when I'm walking somewhere fairly flat (most of the time, my walks are up a pretty steep hill). Since I now have it fairly close, if I notice a difference between Fitbit and MMW, I just tweak the stride length - increase or decrease it by 0.1" at a time - rather than calculating it all over again.

If you're wondering if it is missing steps, I'd do it not by watching the app while you walk, but by reading the number of steps, then putting the app away and walking while counting your steps (at least 100, if not more), then stop, and check the number of steps again. Being off by a few is probably okay, 10% is kind of high.