Castellano vs. Espanol

Some of the recent postings on self-identification of language
communities bring to mind something I've long wondered about.
Argentinians, I know from my own experience, generally call their
language "Castellano". I believe (and here I could be wrong) that
the usual term in the rest of Spanish America is "Espanol".
Is there an explanation for this? Or is it one of those "just the
way things are"?
Dan