This usually happen to me:
Lets say I am runnig a full hd projector.
My mac second display output is matching projector' s resolution.
Projector is positioned so the blue menu display screen is bigger than the mapping surface, so it is just a matter of closing edges in.
But when the mac is connected, mac o.s. second display, which is matching the projector's resolution, is already projecting an image smaller than the projector full capability — the blue screen.
why is that? what am I missing?
tks