Steps vs steps

I carried around a Fitbit for a goodly while. I managed to lose it (not loseit.com) right before my summer trip... It was getting pretty long in tooth with the plastic chipping along the edges anyway so it was due for the most part.I decided at that point to simply rely on my iPhone's M7 or M8 (depending on version of course) to count my steps.The problem is that it really seemed to be undercounting my steps. Not just undercounting but vastly undercount. There was a couple of days that we walked for literally miles (7-10 miles) that I barely got to my 10,000 stop goal.In doing some research I found that the Fitbits are generally regarded as pretty accurate. There's always sources of error, but the Fitbit seems to be close to real.Today starts another little experiment: compare the iPhone to a new Fitbit. The convenient thing is I'll have both on my person so we can have an apples-to-apples (or is that Apples-to-apples) comparison. My hypothesis is that the iPhone is just plain wrong.