John Martellaro for The Mac Observer does a bit of journalism:
Apple Pay shouldn’t normally require an additional electronic signature. However, some users have reported that they’ve also been asked to provide a signature from time to time. This is a first, informal report on when you may or may not have to electronically sign with Apple Pay.
It’s interesting to see how each credit card company handles transactions and their ways of tracking items. I wonder if this is why certain cards seem to work fine everywhere on Apple Pay, while others have some hangups (will scan, but then won’t work, but the physical card will).