DPS is still in the middle of their transition from analog to digital. Being a state wide agency, they will be taking their time as it's expensive - very expensive, which is why patrol isn't all digital all the time. CI was given priority due to the sensitive nature of things and the ability to encrypt (which oddly enough hasnt been fully utilized yet).
That said, digital radios have been issued to all valley motor units and CI units. As DPS gets new vehicles and new rookies, they are given the new radios. Everyone is still operating on analog UHF. The system is only patching CI from analog to digital (full time) at this time. Once in a great while you will hear Central traffic on the AZWINS side, but it's rare. So the units in the field are actually on analog UHF.
For hearing a dispatcher only, and no unit, it's most likely that the dispatcher was covering another district in addition to CI. This happens so other other dispatchers and take a break. You hear this on their analog UHF system regularly. For example, central dispatcher will work central and east until the east dispatcher gets back from break - you will only hear central units, not the east when this happens. CI dispatchers will cover districts for this same reason.