Because the Oat Mtn frequencies are the same as the Culver City frequencies...to reuse the frequencies they have to direct signals so they dont overlap.
Same as some of the Pomona and Burbank frequencies are the same, and the cell in Palmdale reuses frequencies (some verdugos)
For a trip, drive up the 14 fwy and see the Oat Mtn cell and Palmdale cell pop up on different parts of the road, depending on which hill is blocking what signal.
I have heard Glendale units in the SCV responding to a callout. I dont remember if it was a K9 or detective.
Also, Verdugo Fire and its members do have mutual aid agreements that might send them into Santa Clarita. I believe they are the third out Haz Mat responder, as I have heard Burbank HazMat 12 dispatched to I-5/Calgrove before (the FFs didnt think they heard that dispatch correctly!) when LA County's 2 units were busy. Also, during the Truck Tunnel Fire, Glendale's Air Utility was dispatched and beat LAFDs unit to the scene by an hour.
Plus the Verdugo Strike Teams in the future will have a greater ability to communicate back to headquarters (another example would be the strike teams that responded to that Beverly Hills Brush Fire earlier this year)