Alta and WCSOJ have to be built different... Somebody correct me if I'm wrong, but I think WCSOJ is probably more analogous to highrise construction, with open floorplates and a central core, and probably the usual relatively easy to build metal studs and drywall on the interior. Alta is being built as a big concrete grid of apartment units, with lots of cast-in-place walls that take forever to build. Also, WCSOJ is under a deadline to be ready by 2008 and I wouldn't be surprised if Phoenix were paying a handsome premium for that.
Consider an earlier shot of WCSOJ, courtesy of HX: