It seems that there isn't really a consistent AA number for hops. It could vary depending on the vendor or the time of year. Depending on your hop schedule these differences will potentially make a big difference in the final product. How do you work your recipe around this?
Do you adjust your hop totals to get the same IBUs or do you just keep the totals consistent?
How does this effect the later additions where the bitterness is less important than flavor or nose?