Accuracy of iPhone video frame rate

Using one iPhone ("iPhone A"), I made a video of another iPhone ("iPhone B") running the stopwatch app; and when I examine the video frame-by-frame, the stopwatch times vary wildly compared to the frame times. Does this mean the "iPhone A" frame rates are unstable, or is it more likely that the screen on "iPhone B" is just slow to update?

I know this is an unusual question, but would appreciate any thoughts.