Why do I make customers so angry?

I want to know why my personality seems to bring the worst out of customers more than anyone else at the cafe. Well, let me modify that a little bit. I only make people seethe with rage during telephone conversations. In person, though my attitude when dealing with difficult folks is always the same, my pretty, smiling face usually deters them from completely losing it. I smile when I talk to customers over the phone, you better believe it. My vocal stylings definitely took some training. I can be demanding and authoritative, nearly evoking Kathleen Turner's spirit in my commands. I can also be sweet and stupid sounding. There's a great chance that when angry customers talk to me over the phone, they believe they're dealing with an incompetent 16, maybe 13 year old girl. I keep calm on the phone, using my sweet li'l princess voice, which, though I sound like a baby, carries some almost condescending undertones. That's it. It has to be that. What could possibly aggravate an angry customer, calling to complain about her burned latte, than a young and innocent yet belittling personality on the other end of the line, telling her that we're sorry, but we just can't offer her a year's supply of free lattes.
People call the cafe all the time, for all kinds of reasons. To complain, for directions, to order coffee, whatever. Quite often, their requests or random issues are just too ridiculous and I have to tell them no. Did I mention that my cafe is located smack in the center of the state's most wealthy area. I pour cappuccino after cappuccino for over privileged college students, bored and botoxed soccer mom's and their stuffy, new-money husbands. This is my life. So it turns out that the rich and stupid are the ones with the most to complain about. Their lives are actually more difficult than yours or mine. Who would have guessed?
Well, try saying no to one of these, umm, assholes. They don't appreciate it.
Yesterday, a man phoned to request that we ship some gift certificates to him in Florida by Christmas. Shipping gift certificates is not something we do, but I was kind enough to suggest that he leave a message in my boss's voice mail, and maybe when she was back at work in two days, she could consider bending the rules. I gave him no false hope though. I let him know that chances were slim, but he could wait and see.
"That's not okay with me, and I don't appreciate the way this matter is being handled," he growled at me, "I want to you take my order now because I hate waiting!"
"I hate assholes like you," I hissed back. I'm kidding. I would never do that... I was frustrated by this conversation because it felt like the nicer I was, the more furious he grew. Why would a sweet young princess make this troll so unhappy?
Maybe someone out there has the answer I'm looking for.
Good Night Moon!