I have heard and used both the words soro and soror. So its not just a NPHC thing . Truthfully i think what words you use depends on your campus and sometimes your house. And really words like frat and soro, etc. are just slang words and easier to say sometimes. Like someone said, im lazy too. and saying im going to a frat party is just more easier and more common then saying im going to a fraternity party. I mean if you're going to a party, do you say Im going to FIJI, or Im going to Phi Gamma Delta.