I'm not the person that has an answer for you, but, that is a heck of a lot of difference between open (no pressure on the belt) and closed.
If anything I'd expect to see the slack on one side or the other depending on if it was pulling the door closed Vs pulling it open. Unless there is something within the gear drive that would allow it to do that. ???? I know on chain drives the chain will sag a bit on one side in the closed position. Not sure about a belt drive.
You might send an email with pic's to Chamberlain and see what they have to say. You don't want to overtighten it, you may have other problems in the closed position if you do.