Sights in the city

Riddle me this...

Three girls of indeterminate age board the subway. They are clearly travelling as a trio and share a similar sartorial sensibility, jersey dresses, flip-flops and wafty appendages draped over them in an artful manner. They are not dressed for business yet nor are they tourists. I decided that they must be visiting students for a summer program.

But this is not the quandry, for each of these girls, grouped together and chatting about the day gone by, each one of these girls was brandishing a coffee cup, here's the clincher, from a different shop. One had a Starbucks cup, another the classic 'have a nice day' cup of popular legend and the third clutched a cup from Pret a Manger.

How did that happen?