How accurate is the step counter in the health app on the iPhone

I also think it may be very dependent on clothing. I often walk the same path with my wife, mostly level, of just under 4 miles per GPS. Her FitBit on her wrist is very consistent. My iPhone will register sometimes double the actual distance (and number of steps), and sometimes be fairly close. It always counts more steps than her FitBit, even though she is considerably shorter and has a shorter stride. The iPhone is nowhere near consistent with itself. I have the iPhone in my pants pocket, and I have suspected that bouncing is causing it to count more steps. It's kind of a running joke between us- about how many steps I got today. Back pocket seems more accurate than a pocket on cargo pants/ shorts. When she tried her iPhone on an arm mount it was fairly close to her FitBit.