hop flavour can vary inversely with respect to alpha acid content. i.e. High alpha -> low flavour and vice versa.
I think what is being said here is: High AA hops aren't the best flavor/aroma hops and good aroma hops tend to be low AA. There are hops that do well for all three purposes (NB), but try and make an IPA with all Fuggles. At the other end, a Mild made with Cascade would be really strange and you'd add the hops with a tweezers.
The aroma and flavor oils enter the boil within minutes of addition and the oil levels don't vary much year to year. Finally, aroma and flavor are far more qualitative than quantitative, because they stimulate our very complex odor receptors.
Bittering is one note on the tongue. IBU is how loud.