Monthly storage fee discrepancies

I have a variation family of products that is 7 x 3.5 x 1.5 inches, or .0188 cubic feet. Amazon says they charge $.78 of monthly storage fees per cubic foot, which would be $.014. If I enter these measurements into the revenue calculator it gives me an estimate of $.02, but in reality they’re charging me $.28 per month and the support team can’t come to any conclusion about it. Anybody know why that is?