Fitbit flex, active minutes??

I've had my fitbit flex for about a week, I love it!! There are times when I sync it with the iPhone to see where I am with my steps, activity level, etc. there have been times where is shows I have 10 active minutes?? When I check again I have 0??? Today I've been walking for 64 min at a 2.0 pace on the treadmill and it shows 0 active minutes?? It was up to 6, then down to 2, now 0??

I don't even know if it's a big deal bc the steps are being counted!!