Also to add to Roelof's explaination the crank shaft timings can sometimes be adjusted to give a broader engine power by opening the crank shaft much closer to the time the transfer port closes rather than the much earlier opening time in on road engines, the exhaust port timing and the manifold/pipe combo extends the wave reflection time which in turn broadens the power band at the expense of peak power, this also reduces peak RPM thus fuel economy is better in off road than on road.

It's tayloring the power band to the application essentially, a usable broad power band is much more useful in off road, where as peak power and rpm is king in on road.