Starbucks started with "Short", and "Tall" coffee cups. It probably made sense in the context that they were just a few local cafes in the beginning. The problem, of course, is the ever-increasing American appetite. To slake that appetite, a new, 16 oz. size was offered: Grande. Furthering the trend is the 20.5 oz. size, Venti.
"Venti" conveniently skirts the fact that it's bigger than "Grande", but with increasing cup sizes, a "Short" cup just didn't seem to cut it anymore. However, it costs less to the company to continue the size, rather than potentially lose customers, so "Short" remains as a secret, much like the "Misto" (cafe au lait), which also is not on the menu.
The partner (that's what Starbucks baristas are called) that loquacious encountered seems to be an exception to the rule. Most people are rather laid back if working at Starbucks. I let people ask for just about any sizing, and give them what they want. The customer should come before some stupid corporate gimmick.