Actually, this is sort of complicated, but the math shows how it needs to work:
In a "normal" 12 team, two division, 9 game schedule, you play 4 out of division games a year split among 6 teams, so in 6 years, you have 24 total games that splitting among 6 teams, so you're going to play everyone 4 times ( 24/6 = 4 )
But Cal and Stanford play USC and UCLA every year. So look at it from Cal's perspective: they only have 2 out of division games a year to split among 4 teams. So in 6 years, they only have 12 games to split among 4 teams, so they will only play everyone 3 times ( 12/4 = 3 ).
Now, switch to our perspective: we will only play Cal and Stanford 3 times each over this same 6 years. So, for our 24 out of division games, 3 go to Stanford and 3 go to Cal, which means that we are left with 18 games to split among the other 4 North division schools, so we're going to play them 4.5 times each (or really, 4 games against 2 of them and 5 against the other 2) ( 18/4 = 4.5).
That pesky half game in the last calculation means we're looking at a 12 year cycle just to get to whole games against each opponent. But it works out to mean that in 12 years we will play cal and furd 6 times, and Oregon (and the rest of the north) 9 times.
And, that pesky odd number at the end of the last calculation ultimately means the entire cycle takes 24 years to complete with even home/away games before it will start to repeat.