The dots move along the gridlines to show which way a ball would roll down that gridline. If your view is not looking directly parallel to the gridlines, then the moving dots will be showing a component of both the sidehill slope AND the elevation change. With significant elevation change in your putt and a non-parallel view, you can get fast moving dots all due to the elevation change with no sidehill component at all.
It makes it very hard to judge the side slopes when you can't get a parallel view. Best to use forward and reverse view, in chip mode, to try and find the most parallel view. And if you can't find a good view, you have to go on experience and by trying to visualise the effect the elevation change AND side slope combined is having on the moving dots.