2005 75hp merc question

I recently got a 2005 75hp mercury 4 stroke, it is a great engine. I was on the mercury website and brought up the 75hp and the 90hp 4 strokes, they are EXACTLY the same, I mean like right down to the engine weight, bore, stroke, everything. The only difference is that the 75hp says max rpm is between 4500-5000 and the 90hp is from 5500-6000rpm. How do you get the extra RPM out of the 75hp, is it a rev limiter, or a air restriction or what. It just seems kind of crazy that mercury wants almost another thousand dollars for another 1000 rpm from the exact same engine.