What's the expected behavior of erasing with a low alpha value?

Hi

I've been noodling around today and have noticed something odd when I have the alpha value of my erase colour set low: It fades to the erase colour as expected but then stops just short, leaving ghost images of what was originally there.

Is this expected behaviour? It's a long time since I've really played with Jitter, but I can't remember seeing this happen before. I've tried in Max 6 & 8 both with the same results. Sample patch:

Thanks :)