The jump is occurring because the end of that Roto Brush span has been reached.
Please, please, watch the tutorials and read the documentation before trying to use Roto Brush. It is not a beginner tool; it is not an intuitive tool; it is one of the most advanced and tricky tools in the application.
Thanks for both answers including on the creative cow.
I did watch 2 tuts about rotobrush and don't recall any mention of span... but I see in yours there is quite a lot more detail.
Oddly enough I did manage to get the rotobrushing I was after before seeing this post or any other information other than by experiment.
I had been advancing one frame at a time with no corrections mostly when I noticed the span thing coming into play.
I assumed a frame by frame would give the best chance for AE to do whatever it does. And as I mentioned in OP I was getting lots of jumps.
I can say though that I found just hitting play stopped most of the problem. I was able to get thru the task much quicking by hitting play. Maybe the span thing doesn't apply then, because it seemed to move thru more than twenty frames.