refill?

Once in a while someone would come into the caffe with a 7-11 cup, a paper cup from somewhere else, or their own mug and asked for a refill.

I always wonder: how would there be a "refill" when there had not been a purchased cup to start with?

We make everything at the caffe by the cup, so there's no refills anyhow. It's just interesting to hear people use the word "refill" like this sometimes.