I'm no scientist, but it seems possible that having a very strong core could make it more difficult to have a flexible back. These two muscle groups work in opposition to each other, no? Again, this is speculation as I'm not an expert on anatomy or anything, but it makes sense in my head.
....It's interesting that this would be my first post after years of lurking, but I suppose stranger things have happened.
Welcome to posting Jackie! Post long & often in the future!