While we never see the other half of Garrovick's cabin, we see enough
to know that the space was there (complete with rotating wardrobe and door out into the corridor). The mesh divider does not seal off Garrovick's side in any way, shape or form - which is a shame really, as all it would take is a wild wall panel to plug the gap!
Maybe there is an additional (unseen) bed in the area behind the mesh, but I know that if I was forced to share a bedroom with someone else for any length of time, I would prefer to have some of that space set up where I could sit and work/read/relax etc. A bunk bed would free up some of the room nicely.
Maybe Garrovick and his roommate just couldn't agree on who got the bottom bunk, and this was their solution?
Ever since I saw "The Undiscovered Country" I didn't mind thinking that on the TOS ship the briefing room sized cabins had bunk beds for 4-6 enlisted crew. The junior officers like Garrovick may or may not have shared cabins while senior officers got their own. Lucky ones got their own bathroom. But that's just IMHO.