There must be a Zeta Phi Beta/ZTA Bond that we don't know about
At my alma mater, the NPHC Zetas actually assisted the NPC Zetas with learning steps for a step show. I think my sorors came in first place, and your sisters came in second!
One of my closest high school friends became a ZTA, and I became a Zeta. I used to work in the Student Government office on campus, and one of the senators was a member of ZTA - and whenever we saw each other, we'd greet each other by saying "What's up, Zeta?" or "What's up Z?" A couple of times we even said "What's up, Blue?" since we both have blue in our sorority colors.