It's my understanding that the 33hp of the 60's was the re-incarnation of the older 35hp, and the 28hp was the re-incarnation of the older 30hp. Were changes actually made to lower the horsepower in each case, or were they just rated lower?
All marketing, basically to better fill the gap between them and the next offerings in the line. They dropped the 28 when they started offering the 22-cube 25hp, and replaced the 28 with the 33.