From what I've read of the established backstory, Zeta Halo was built out of one of the original 12 Halo rings but shrunken down in size and made part of the later 7 Halo rings we know and love, and is consequently both older and structurally different from all the others. I wouldn't be surprised if all the stuff about the Endless was just the beginning of what secrets/surprises this ring contained.

And it turns out that the Greek Letter Zeta is associated with the number Se7en so I have to give it to 343 for carrying out an old Bungie tradition here ;)