Good questions about what you are hearing, and the reasons are not obvious.
Many/most frequencies are available at more than one transmitter/receiver site. The controller can select which site to use, probably now with a tap or two on a touch screen. Reasons for changing include broken or degraded equipment, temporary interference from external sources, routine (e.g. daily) operational tests, airplanes at low altitudes on opposite ends of a large sector, and reducing the number of frequencies needed for adequate coverage when sectors are combined. In addition to all the other details, controllers need a mental model of how frequency coverage works in their sectors, especially in mountainous areas because VHF is line of sight.
This document discusses some aspects of communications, and defines terms such as RCAG, RTR, and RCO:
https://www.faa.gov/documentLibrary/media/Order/FAA_Order_JO_6500.28.pdfIf you do a general search on RCAG you can probably find information about current Center sites.
I have a list of sites that is about 20 years old. At that time, ZLA31, 126.775, had sites at Julian and Yuma. Those may have moved or there may be additional ones now. If I mouse-over the archive access link on the LiveATC page, my browser shows a link ending in "ksan_zla31", which suggests that the feed provider is near San Diego. My wild guess, 20 years ago, would be that if the Julian site is in use you can hear the controller.
A controller can transmit and receive on one or more frequencies as desired. Some sectors are so large that more than one frequency needs to be used even when operating as a single sector. If combined sectors cover a small geographical area, it might be possible to have everybody on one frequency. At the time sectors are combined or split, the controller can send an internal message to surrounding sectors telling what frequency or frequencies to use.
There would be lots of possible configurations, and your observations about 09+30 and 25+28 are not surprising to me, and now hopefully not to you. Another thing to note is it takes some time for signals to get from the feed provider through the internet to you, and this can vary from a few seconds to 30 seconds or more. If you are listening to multiple frequencies used by a single controller, you might hear an airplane response on feed "A" before you hear the controller give the instruction on feed "B".
Bob