Why is one portion of the Rotobrush span line green, other gray?

I am trying to figure out how to do a full rotobrush analysis with one click.  In a given clip I have 1300 frames. Baseframe is frame 620. If cursor is at frame 620 and I hit spacebar then I see the clip going from frame 620 to end, and the span bar will become green as it is moving forward.  THen when it gets to end I see the span bar becoming green starting at frame 620 to the beginning.  At the end, it seems the analysis is all done. But only one half of the span bar remains shown in green. Like this:

Why is that? Is this a glitch? The mask seems to have been propogated everywhere, so I just don't understand why there is this visual cue showing the span bar green only for the backwards direction analyzed frames.

Ideas?