Agree, I also think you can handle credit fine, but they think that a mortgage is evidence of credit-worthiness. Also though, they want to see how you can manage risk and aren't interested in how you manage cash.

If you really want to increase your score, you can buy an official FICO score and a score report which will give you some information on how you can improve your score (or more accurately, they will give some general statements on how they got to your current score, and you have to figure out whether it can be increased and how to do it), but it is becoming more difficult to find because other companies are selling scores that are not actually FICO scores but are branded as them and have much less helpful advice. If you are going to buy an official score I might drop by a FICO forum first to make sure you are buying the right one.