I think you have to really compare apples to apples. I'm pricing out MUC- to dc area in 2/20 and the taxes for United range anywhere from $97-$120, whereas AA is fairly consistently $103 (*rounding off all fees). I believe the differences are the nonstop for United/Lufthansa are the $97, whereas the other United award flights AND AA have stopovers so that apparently adds to the fees. Each country appears to have a specific tax amount and origination in European countries appear to be higher (some much) than ours.
Looking at the same date in February on a couple of other destination goals- if you fly from PHL-DUB on AA and BOS-DUB on United (both nonstop on their respective planes), the taxes both price out at $5.60. Looking at American Airlines to Venice, most taxes are $5.60 if nonstop whereas if you fly United to Venice there are no tickets on United branded flights (only code shares), and each itinerary has a connection. The taxes fluctuate depending on the airlines flown and the countries of the connection so it really does depend on origination, layover (if any) and the country of origin of the airline company you're flying (aka "metal").
Hope this makes some sense!