So I am somewhat confused

I had installed FBlit and to my recollection, thats the only thing Ive messed with.
When starting UFO, it gave me a garbled screen. I pruned away the extra things in the Monitors drawer and this gave me the image back, but it was too large to fit the screen.
On a whim, I unplugged the external scan doubler thing I use, and plugged the VGA monitor straight into the adapter I have in the amiga's monitor port and voila... UFO displayed a crisp nice image, straight to my LCD monitor, with no scan doubling.
Exiting to workbench, and the "input not supported" message came back until I connected the scan doubler again.
What gives ?