My wife & I both have excellent credit scores, and we pay all of our bills in full every month. Nevertheless, I've noticed a ~50 point swing in credit scores from month-to-month, all dependent on whether airline tickets, furniture, or charitable contributions happened to make it onto this month's bill. Our debt-to-liquid-assets ratio is something like 0.1%, so there's never any real risk of not having money to pay it off, but of course the credit bureaus don't have information about our assets, so they evaluate us against what other people our age have, which (being Millenials) is not very much.
Knowing how the system works, we can take steps to game it, like not putting any major purchases on credit card in the 3-6 months before getting a mortgage. But still, it's slightly ridiculous that something that's supposed to measure your creditworthiness can swing so much over short time periods.