If you check through the latest BJCP style guidelines you'll know that there are things like IPA and then a new style called 'American IPA' because it is so far from being an IPA, that instead of brewing within the style guidelines, they've made their own style and put American in front of it.
The word imperial has nothing to do with high gravity, although closely associated with the 'Imperial tag, I will agree. Imperial beers owe that name to the first Imperial beer, which was the Imperial Stout, brewed in England for the Russian Imperial Courts in I believe the mid 1700's - but I could be way off there.
For reference, I bloody love American IPA's - I even brew an American Imperial Red Ale (according to style) - I just find the term a bit funny!