Here's the Tri-Sig versus Tri-Sigma thread.
http://www.greekchat.com/gcforums/sh...&threadid=8388
A lot of times the national will try to get a "unified" name used, but it has varying degrees of success (AGD/Alpha Gam, ASA/Alpha Sig, SDT/Sig Delts). It's like naming a baby Elizabeth - one person will call her Liz, another Beth, another Betsy, another Izzy. If the chapter is happy with the nickname, I really don't see what the difference is.