I have one also and I’ve been wondering the same thing. The main thing I’ve come up with is the cylinder block is different. It has blind cylinders (like Merc) and I wonder how much difference there is in combustion chamber and ports design. I’ve been meaning to pull the carb and compare with the 5.5, but haven’t been inspired enough to get on with it.
One thing I do know, it has considerably less power than the older Gale 5hp wedge head motors, which could run circles around it. But they could run circles around the 5hp Johnson TN-28 that I had also.
But it is very quiet. So quiet that it seems puny.