To skim means to remove "something" from the top(usually referring to liquids). Which makes sense to use to refer to a device that sits atop a card reader.

https://www.google.com/search?q=card+skimmer&safe=off&source...

One thing to remember is that keyloggers have been around much longer than card skimmers, keylogger is a well known and well defined term.

http://en.wikipedia.org/wiki/Hardware_keylogger

https://www.google.com/search?q=hardware+keylogger&safe=off&...

It's all very interesting to watch as criminals become more sophisticated.