Perpendiculat snap to polyline

I have a polyline having 3 segments (line, a short arc, line). Want to measure distance from given point to the plolyline. - click the point/ select Perpendicular snap/ move cursor over the polyline => ICAd often shows the point snap quite far from the polyline. It depends on the actual position of the cursor along the polyline - only when the cursor is VERY close to the correct "perp" point, Icad selects the right point. Is this a bug or is it considered normal? Regards. Vaclav