Yeah it's just a fun quirk of how the UC system started and expanded.

The state established Berkeley as part of the UC system, but California State Normal School at Los Angeles (what a name) was it's own independent university system that got merged into the UC system to become UCLA.

Then because UCLA became the bigger/wealthier campus for a period, it demanded independence instead of being governed by Berkeley. So it got co-flagship status. Santa Barbara State College then got acquired to become UCSB, also granted independence. So you could sorta say those 3 were the original main campuses.

Then the UC system became the UC system we know today with the UC system planning campuses from scratch. But since the 3 universities in the system were all independent, they just kept planning the later campuses to be that way.