i think you also need to take into effect how the cars with the given engines are driven.

minivans arent typically run very hard...most people who buy a minivan arent looking for performance...and when a vehicle isnt beaten, the engine will last alot longer. they also arent under much stress such in hard acceleration, repeated high rpms, etc. im sure that some people run their minivans hard, but typically, you arent going to see many hot rodding minivans...so the engines arent stressed as much...

take a mustang GT, and 100,000 miles. the odds are pretty good that the mustang engine has seen more rpms, especially hig rpms, in 100,000 miles than a minivan might have in 150,000 miles...other things too, like long distances at high speed and high rpms..over-revving, etc.