As I've started my 100 day challenge, I thought I'd do a quick comparison of the data from my pedometer and from my activity monitor. I use a Polar A360. I've posted some stuff about it before, so I won't go into too much detail here.
The A360 takes all your activity and converts it into "equivalent steps". I'm guessing it uses some sort of algorithm to do this. So, when I play tennis for example, it will show a higher step count than when I go for a walk, not because I've necessarily taken more steps but because of the way it converts movement into steps. This shows up in the statistics. Interestingly, one day the A360 under-recorded steps compared to the pedometer rather than over-recording (although over and under recording aren't exactly the best way to describe what it does!).
Anyway, using the 14 days from August, here's what the comparative data looks like:
Total Steps: 192525
Average per day: 13751
Total Steps: 233407
Average per day: 16672
Interestingly, the day the A360 recorded fewer steps than the pedometer was the day I intentionally went for a walk and set my wristband to monitor that activity (i.e. to log the walk as a training event). It's also partly why I'm undecided about how best to monitor myself during the 100 days. At the moment I simply use my pedometer and make sure it hits 10k each day.
I guess in the end the data just underlines that you need to decide what you want to measure and then choose the appropriate method and equipment to do that job.