To me, it looks like someone tried to replace the window inside the plastic frame. Since the other windows are crazed and this one is not; it also seems plausible. So what they did was cut the plastic frame off of the old window; install new lexan and then tried to re-bond the lexan to the old frame with silicone. The gap you see is where they cut the frame be able to remove the lexan.
These windows with a plastic surround are usually made at a factory that injection molds the frame material around the lexan; so that it seals perfectly around the window and there are no seams. It's not just a simple matter of slapping in a new piece of lexan when the plastic crazes.
Your best bet for getting it repaired properly is to buy a new portlight from a company that makes them like Mark Plastics in Corona, CA. (IIRC)