I'm from Boston. When I was growing up, if you asked for a Coke, you'd get a Coca-Cola. If you asked, "What kind of soda do you have?" you'd be told "Vanilla, chocolate or strawberry," because a "soda" is a drink made with carbonated water, flavored syrup and ice cream. If you wanted to know what your choices in carbonated soft drinks were, you'd ask, "What kind of tonic do you have?"
In other parts of the country, people hear "tonic" and they think "tonic water," which is that quinine-flavored stuff you make a gin and tonic with, or a hair product, or some kind of medicinal concoction. Carbonated soft drinks are soda, or pop, or coke. A college friend from the South explained to me about "coke" as a generic term and said it was a pride of place thing -- the Coca-Cola company is headquartered in Atlanta.
It sounds weird to my Northern ears, but so do "sack" for bag (southern U.S.) and "loo" for bathroom (U.K.). Linguistic differences are fun!