I've seen something like that on a neighbor's horse I used to ride. I think the problem was he was a bit thin, or at least his spine was more prominent, and that the back of the saddle rubbed there.
I saw a similar bump on my Foxtrotter when I first bought her (because she was also underweight) but when she fattened up, it is no longer visible.
So I dunno, I guess that's not much help, is it? Just make sure your saddle doesn't contact the spine in that area.
I am not aware of any problems caused by a bump like that. Just make sure your saddle doesn't aggravate it, but otherwise I think is more of a blemish than anything serious. Just my opinion of course, because I'm not a vet.