yep, could be several factors, the Kaibab trophies are said to have abundance of calcium.
AZ elk, esp WM Apache are sought after because their spreads are the largest. One thing I always thought contributed to their ability to grow such spreads is the spacing of forest in AZ vs other Rocky Mtn states. Some places I hunted elk in CO (like the Medicine Bows) amazed me that they could traverse through, pines so close together...probably just genetics though.