Well, I don't know. But if I had to hazard a guess, they likely detuned it as cheaply as possible. ANd nothing is cheaper than a separate computer map. We do it all the time in the car world - my 2014 Jetta GLI is a 210 hp car from the factory, but with a tune, 250 - 260 is more realistic.
So that would be where I'd start looking. It's possible the 250 is running a different cam and other internals, but an enterprising individual can probably figure it out if they really wanted to.
The HP rating is at different RPM's . But a Computer Map is the cheapest way.... Since the whole engine was purpose built by mercury, it making two different cams, or other parts make little sense as the key is to produce in high volumes to keep cost down.