The distortion is because the AE comp window is doing an anamorphic display of the comp. The project is set to be the proper size for SD video, but its cheating the pixels so that when the SD signal is stretched across a 16:9 display, it will look how it supposed to. Viewing what you did on a 16:9 production monitor.
That is all based on speculation of shooting SD, with SD’s non square pixels. HD has true square pixels, very little problems with that.