Windows flickering

I understand that Windows does not double buffer screen contents, but why is it that if I enlarge a window the contents, i.e. a transparent label, flickers like hell, and when I move the window no flicker at all?

When you’re Enlarging the window, the contents may also need redrawing as they change sizes (even if you don’t have them set to do so). Moving the window probably doesn’t cause the contents to redraw at all.