Typically that trim piece is nailed onto the face of the surrounding wall, so if the edge of frame is not flush (frame is not as wide as the wall) then there will be a gap - it shouldn't be a functional issue though. As far as fixing it you'd need to shift the frame over which would be pretty big job and then you might have a gap on the other side of frame, lol. But also, is that a metal frame? It looks there is a bit of welding, which if that's the case, that's just the way it was put together and would be impractical to repair. I guess if it really bothers you, you could fill in gap with bondo and repaint it, but don't think that would be worth it.