Mel have you ever taken a measuring tape to your boat? You may be surprised at what you find.
I have a Silverton 310 Express Cruiser. One may think that it is 31' long, but in fact it measures 33' 6" stem to stern. Now if you go RV'ing in a boat to visiting marina or even if you keep your boat in a home marina they will go out and measure every inch of length and charge you according. Does not matter what your model #'s are.
Every manufacturers model numbers seem to be close to what the length is, but I believe both boats and RV's play around with those numbers, and they are so important to know the truth especially when you are pulling that BIG box down a highway or under an overpass.