I've always made it a practice to include the Interop frequencies as well as a private-call wildcard for the sites I monitor. Several days ago Chandler called in for Ranger Air support, and sure enough, clear as day, they were communicating (Ground to Air) on the Interop talk groups. You'd be surprised what you hear on them from time to time.
Like the previous response stated, it's probably just your programming or location.
Edit -
I'm curious, which feed are you listening to? I don't see one for Glendale.