The sim reads the attach position from the airplane exit config files, but that doesn't mean that any jetway would automatically always dock there, no matter what, because it depends by many factors:
- how the jetway is made and its constraints
- how it's positioned in the scenery
- how your airplane is positioned in relationship to it
We can only say that, if you constantly see ALL jetways at ALL airports always docking at the wrong height, is likely a problem with the airplane itself. If it only happens on some airports, is likely a problem of the jetway model.