Hi Pete,
That's because on sequence preview next frame will stop at the closest frame on any track. Say if you have two clips with different speed on two tracks, you are getting some thing like this:
Track A: 1 2 3 4 ...
Track B: 1 2 3 4 ...
Since the frames are not aligned on the timeline, the next frame start from the beginning in the mixed sequence is something like this:
A(1)+B(1) A(1)+B(2) A(2)+B(2) A(2)+B(3) A(3)+B(3) ....
So that's why the number is not jumping every time you click 'Next Frame'.
This is not an issue since VideoPad will insert and delete frames at the right position depend on user settings when export.
Hope that make sense.