This explains a lot, American microwaves have these settings for different types of food etc, it seems most people throw something in and just 'nuke it'. European microwave ovens on the other hand, have a setting for different wattages (90W up to 720W 'Max' in my case), which, combined with instructions in the recipe or on the box, provide the right setting for this particular food.
American microwaves almost always have a fairly easy way to change the power level, it's just nobody bothers to use it.
Are you sure that European microwaves actually use continuous power at those wattages and not also "simulate" the wattage by using short bursts of a fixed power?