San Mateo County GREEN (CWMA) bleed over

I regularly monitor the San Mateo County green channel (488.8875, PL of 114.8) and once in a rare while I hear a familiar transmission come across that the county dispatcher is unsure of and asks the unit to identify. There is never a reply because the bleed over only lasts for a single transmission. It's a familiar transmission to me as I used to work at VTA (Santa Clara Valley Transportation Authority) and recognize it as a bus operator transmitting to OCC (operations control center). When I first heard this a few months ago I looked up the frequency VTA was using (488.8375, PL of 114.8). My best guess is either one of the VTA radios is out of tune and occasionally tripping the repeater at the south end of San Mateo County, or perhaps that southern repeater itself is out of tune. Whichever the case it is such a rare occurrence that I doubt anybody has looked into it. What I'm wondering is why do two neighboring county agencies (one a sheriffs department and one a transportation agency) share the same PL tone, especially when their frequencies are so close?