I think IHQ names chapters according to what Area or Province the colony is in.

The chapters that begin with "Delta" are located in Hawaii, Washington, California, Arizona, Nevada, Idaho, Alberta, British Columbia.

The only exceptions are Iota and Phi chapter at UW and UO, respectively.

The "Zeta" chapters are more toward the north and/or east, with Michigan, Maryland, Mass, RPI in NY, parts of PA, and Ontario included.