Well, it does seem weird though. I have other acropora at that same level (two purple tip tan corals, green that you can see on the left, and a tort) and they look fine. I also have some montipora that is doing well, some even quite a bit lower. I have some hydno that is darn near the sand bed and they are growing quickly as well.
I guess as long as everything is growing I shouldn't worry too much about it.