It has to do with some of the basics of the beer.
American pale ales are 4.5-6.2% ABV, with IBUs of 30-45 with some clean malt character and often with some restrained caramel notes. It's got a definite hops flavor and aroma, but not really more than moderately hoppy.
American IPAs are 5.5-7.5% ABV, with IBUs of 40-70. They have a prominent to intense hops aroma and flavor, with firm bitterness that may linger. There is less malt character, and actually should be quite low and should finish dry and crisp. There may be a bit of caramel note, but not as much as in a pale ale.
Generally, American IPAs have more alcohol, more bitterness, and more hop notes. They can actually be quite bitter and hoppy, while an APA will generally be less bitter and hoppy.