OP, how do your classic EX8 and EX2 compare? Whether you do or don't see a similar difference might indicate whether the gap between your AU scores is due to a difference in those algos specifically or something broader in FICO 98 vs FICO8.
It looks like you do have a similar spread between your classic EX8 & EX2, so the difference in your AU scores may be due to general differences between 98 & 8. I don't really know enough about FICO 98 or your profile to speculate as to what those might be. With regards to the reason codes on your report, both "many new accounts" (on EX AU2) and "short account history" (on EX AU8) look accurate from what you've disclosed. The reason code can show up if you're missing any points at all under that metric. I know from my own case that on EX2 industry options "many" new accounts means more than none (though I don't know for how long, I believe it's more than a year), so 2 would qualify, despite how they word the reason statement. On FICO 8, I think one thing "short account history" references is AAoA, and you're under the 7.5 year threshold that's believed to be the max for that metric.