Why does the frame count not exactly match?

The total frame count accuracy over a longer period of time is limited by the absolute timing  accuracy of the incoming signal. For example: When concatenating 21 hour files recorded from an SDI signal together, it may result in a total duration 38 frames (1.52s) longer than the expected 21 hours. Considering the accuracy of the  free running SDI clock is limited by crystal reference. In general used crystals have an accuracy of 20 ... 50ppm. 38 surplus frames over 21 hours translates to: (38*0.04) / (21*3600) = 0.000020106 = 20.1 ppm. This is within the expected error margin.