Credit scores are odd ducks if you are debt free. If you are debt free you likely have a single, or maybe a couple of credit cards that you use but pay off each month. If you use that(let's say 1 card) for everything but pay it off, it can impact your credit score by showing you as using a sizeable chunk of your available credit. Depending on when, during the CC billing cycle, they run the credit check, it could show you as using 3K of your available 7.5k of available credit.