My DS8 would add like this, especially when we first started giving him two digit numbers. He would explain how he got the answer and it always seemed complicated to me. I never worried too much though, since a) he was coming up with the correct answer and b) I thought it was a good thing that he made simple things more challenging since it gave his brain a work out it never got in school.
My DH is always advocating the subtract 1 rule when dealing with 9. I keep working on the "know by heart" method, since that is easier for me
