They do vary from harvest to harvest, that is why they are listed on the package. What I do in Beersmith is to see what the desired IBUs are for a given recipe (using the AA% already listed) and then in the inventory area change the hops to what I have available. I then adjust my hops to match the IBUs desired. Usually, it's really a negligible amount. I mean, the different between 6.9% and 7.2% doesn't matter to me a bit. One time, though, I was expecting 4.8% tettnanger, and got something like 2.2% and ended up not having enough hops!
Usually, the "high" AAU hops are within a percent or two. I adjust them to get the desired IBUs. The lower AAU hops are usually (but not always) for flavoring and aroma, so I don't really worry about those.
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