I have a horsepower question?

i have a horsepower question i think. something i have always wondered is how can a car with 170 hp(pontiac 2004 grand am ram air) have a speedometer that says 150 mph. when another car(2000 pontiac grand prix gt) have 200 hp when the speedometer reads 110. then the same pontiac grand prix but a gtp have 240 hp and have the same speedometer at 110 mph. is it just a way to sell it saying hey look it goes this fast or am i missing something? its always been confusing me and i have never figured it out.