In this previous question, soulblazer and TaW both offered solutions and provided screenshots, for creating rounded button corners.
Rounded edges in button C# (WinForms)
Both of these screenshots show that the top-left corner is well-rounded, the top-right and bottom-left corners slightly less so, and the bottom-right corner less still.
I can't see anything in the logic that should cause this behaviour but I've had the same experience. Is this just a quirk of how Winforms are displayed or is there some way around it?