Somewhat unrelated, but it look me a good 5 minutes last week to figure out that some ticket machines in Munich would only take credit cards for certain kinds of tickets. For example, you could pay for an MVV ticket using a credit card on MVV machines, and for a DB ticket on DB machines, but DB machines would only accept cash for MVV tickets. The only indication of this was a very faint icon on the screen.

So, maybe you're onto something when you said that only certain machines actually accept credit, despite being externally marked otherwise.