I didn't use exact numbers for the beam but the first boat rates for a 120hp. The second, 105hp.
I'll give you a better example. Champion sells their 183 line boat in both an entry level CX version and the upper end Elite version. Same boat, same identical hull. The CX is rated for a 150, the Elite at 175hp.
Why?
I've asked the question at *** on the Champion board and never gotten a straight answer. Those guys just bash you for asking questions. In essence, if you want the 18 foot boat from Champion and want the thing with the max HP motor for the best overall performance you're forced to buy the elite version of the boat to get it whether you want all the "Elite" bling or not.
I still say the reason for under HP rating these boats is to create the illusion that the buyer is getting more boat for less.
What's so funny is how the industry has changed from the way it was years ago. Back in the day, manufacturers used creative ways to measure boat beams (usually at the transom) in order to boost the HP rating on a typical bas boat. I'm sure you remember the old Skeeter Wranglers, Champion Super V's and the boats by Hurst. Those were little 16 foot jobs with a transom beam measurement of around 85". All of those boats only figured out to around 140hp for a max rating. Unless you didn't measure straight across at the back end of the transom but measured from the back corners, out to the setback and then back again to the other side. Suddenly max hp comes in at 150HP!! ;D
Not illegal to do it that way though it was not in the spirit of how the measuring method was to be done.