Well, the Earth is round, so the only question is why the one image made the horizon appear straight. If I recall correctly, that was the camera on the second stage, which may have had a different field of view. As someone already suggested, if the field of view was small (but magnified), the Earth curvature could look flat. (I recall the engine assembly blocking the center of the horizon span, which may have contributed to the appearance by omitting the center bulge.) My own hunch is that the exhaust plume was refracting the image slightly