Soda vs. Pop: What Do You Call It?

Linguistics have always fascinated me. Why does the phrase "ya'll" roll off of my friend from Atlanta's tongue, but forced and awkward coming from mine? I just can't get away with it, and instead opt for the more formal, drawn out "you all" like most of my other New York counterparts. Similarly, why do I refer to a long sandwich as a sub when my friends just an hour or two south say hoagie or grinder?