Me and my friends sort of have a theory that I posted last year ( we initially thought they they favored applicants with higher stats over applicants with lower stats but more ECs). A ton of us that were accepted to SF, didn't even get an invite from San Diego. Personally, they rejected me two years in the row.
I think they are just looking for different things in people. At first I thought it was very low GPA that got me rejected - ~ 3.3, but then they invited and accepted someone with like a 3.0 something last year. Now I'm just clueless.