Cloth doesn’t use Pinning by default, but you can enable it and then ‘pin’ a Vertex Group. Like when you want to make a flag on a pole,blowing in the wind, or a piece of cloth hanging to dry from a washing line.
When Blender (or any software probably) had to calculate those ‘fixed’ Vertices when the cloth (Origin) would be moving, that would be complicated and unnecessary, while they (pinned Vertices) are not moving, but they would be in a different position compared to the Origin…
I hope that makes a bit of sense (at least intuitively).
