We do have a lot of problems with fake notes in various currencies, unfortunately. There is the possibility that the crew member suspected your notes weren't quite "kosher", but decided to not accuse you of carrying fake money? I'm not saying you would have fake notes, by the way

Next time, try to get smaller notes. I always have trouble spending $100 bills in American stores, so I won't take anything higher than $50 when I get my currency exchanged.