I was impacted by this issue in many marinas as well. The problem as I recall is due to a formula that USCG uses to compute your Gross and Net. The formula is based on the length, width, and depth. You have to fudge the dimensions to get the weight figures to come out right. As the length is most important to most non-weight considering marinas, you need to fudge the width and/or depth numbers.
Nobody ever checked my fudged numbers on the width and depth and I never had to overpay for a lighter boat again.
Circled the world 2011-2017