为什么我的石英弧显示比直线更粗？

I'm trying to create a custom UIButton that should look like a UIButtonTypeRoundedRect. In my drawRect:, I'm creating a path with first one call to CGContextMoveToPoint() and then four calls to CGContextAddArc(). I then stroke the path. However, in the resulting image the four rounded corners are clearly thicker than the rest of the path.I'm trying to create a custom UIButton that sho