ok I shall explain why.
when you insure a boat and engine , sometimes the boat might only be allowed 10hp maximum on transom !!!
this way you can have this size engine that is 10hp max at say 5,000 rpm but with good power upto 5,000 rpm .
if you have a boat that can have 15 hp then you buy it as 15hp and can run at maybe 6,000 rpm.
it is cheaper to inure 10hp than 15hp....and some lakes only allow 10hp.
mercury used to sell a 4hp , 5hp and 6hp outboard engine, all the same motor but different carburettor and jets etc.
to make the 4 into 6hp was bigger carb and then swing bigger propeller.
mercury make 50 and 60hp that is same motor now, and Honda 50hp is same engine Honda 60hp, 2 minutes to change for
the extra 10hp. just simple job,
so it makes sense to make more horse power range from 1 engine.
ford escort 1600 L and ford escort 1600 GT...same engine, big difference in power .... 1600L much cheaper to insure.