Ahhh, see that's what got you, assuming that the arch was the same size as the others. If you look closely at screenshots, the flat part at the top of the arch facing the alcove is about half the width that it is for the rest of the kbeam corridor. The archway itself I think they just cut a piece of material out of one of the kbeams at the top, stitched them back together and stuck the "narrowed" version at the hallway to the turbolift junction.
From a distance, it doesn't look any different, but in the close ups you can clearly see the spacing is a lot smaller.