The attached file makes it obvious. There are two polylines:
- The left one consists of a 90° arc and a straight line.
- The right one uses 10 segments to approximate that arc in addition to the vertical straight.
- The line type of the whole layer is set to dash-dot.
- But the right one doesn't really show that, because the pattern restarts after every single node.
That's a behaviour I would expect from independent line segments. But a polyline IMHO is supposed to show such pattern continuously.