tl;dr version:
yes, you are right, there are seasonal variations in a hop's aromatic qualities, and AA variations which necessitate adjusting the quantity of a given hop addition gives rise to even more variation. However, in practice this is usually quite insignificant. If you are concerned that a considerable AA variation requires significant adjustments in a late addition of a very aromatic hop, one option to deal with this is not to vary the aromatic hop addition, but make adjustments in your neutral bittering additions which have far less of an effect in the finished beer's aroma and delicate flavors.
Details:
Correcting bittering for different AA percentages is easy; it's simply a matter of multiplying and dividing. If your recipe specifies a 5% AA hop and yours is 6%, you use 5/6th of the quantity in the recipe to arrive at the same bitterness. You know the drill. :)
The aroma properties of a hop are an entirely different matter. There are literally hundreds of compounds involved in producing the hop's flavor and aroma profile. The bulk of these (by weight and volume, not by number) are known as hop oils. Part of the hop oils (20-50% or so) consists of oxygenated hydrocarbons which are water soluble and play a large role in dry hopping. Other parts of the hop oils are not water soluble.
Since the exact composition of hops is mindbogglingly complex (literature cites some 400 aromatic compounds) and the effect and interaction of all of these is not fully understood, specifying seasonal variations in the aromatic qualities of hops is impossible in practice. A comprehensive certificate of analysis can give you a fair indication at best, and you'll have to know how to read it in the first place. Fortunately the genetic makeup of the hop (i.e. the hop strain) is the most important factor, and seasonal variations play a secondary (although detectable) role.
Therefore in practice most brewers simply assume the hops aroma profile to be fairly constant. Which means that if you increase or decrease your hop additions to compensate for AA variations, you increase and decrease their contribution to the beer's aroma and flavor profile accordingly.
Whether or not this is a big issue depends on how you brew. Varying the addition of a very aromatic bittering hop that is added mid-boil will have a much larger effect that varying the addition of a neutral bittering hop that spends an hour in the boil.
To summarize: yes, you are right, there are seasonal variations in a hop's aromatic qualities, and AA variations which necessitate adjusting the quantity of a given hop addition gives rise to even more variation. However, in practice this is usually quite insignificant.
If you are concerned that a considerable AA variation requires significant adjustments in a late addition of a very aromatic hop, one option to deal with this is not to vary the aromatic hop addition, but make adjustments in your neutral bittering additions which have far less of an effect in the finished beer's aroma and delicate flavor profiles.