I believe Wayne is correct. Large transit agencies all over the state love these "Request-To-Talk" (sometimes called "Transit Trunking") systems. There's really nothing else like them outside of public transit (because I assume no other radio users would want those features).
After riding San Diego MTS a lot and observing the system, I wrote up a thread on it here: ScanDiego.com • View topic - MTS/NCTD 800 MHz digital radio system
Basically the driver has a telephone style handset and an MDT. The MDT is an intermediary between the driver and the radio on the bus. If the driver wants to call dispatch, they press RTT (Request to Talk) and they get put into a callback queue. If they have an emergency they can hit PRTT (Priority Request to Talk) and bypass the queue. When the dispatcher answers a 90 second timer is started, and if the conversation exceeds that time the call is automatically dropped. The MDT provides other functionality, like pre-selected text messaging to report delays, wheelchair passengers and mechanical problems, as well as allowing the drivers to automatically log on and log off.
Most transit agencies use analog conventional for these systems, with the first channel carrying the MDT data and the subsequent channels being given generic labels like "Bus Voice 1", "Bus Voice 2", etc (In norcal, Golden Gate Transit, AC Transit, SF MUNI and more can all be found on 480 MHz with this type of set up). Here in San Diego, both MTS and NCTD do the same thing but they use conventional 800 MHz repeaters with digital modulation.
I would not be at all surprised if LA MTA is using this same setup for their buses. If that were the case, each of those talkgroups you identified would simply be arbitrarily labeled as "Bus 1", "Bus 2", etc. It would explain why there's no rhyme or reason to what you're hearing, because the MDT "Request to Talk" system is doing the talkgroup assignments. While the MDTs can use cellular architecture, almost every one of these agencies I've seen has a conventional data channel for the MDT downlink/uplink. It may be worth checking MTA's 900 MHz licensed frequencies and see if you can find one.
Edit: Thanks for the plug Brian!