I'm not really sure that's the reason. The Pac-12 actually won a lower percentage of its OOC games than the Big 12, but its conference RPI is higher because (evidently) it played a tougher collective OOC schedule. SOS accounts for 75% of the RPI, the win-loss record only 25%.
The calculations all vary a little, but
SOS is typically determined as a team's opponents winning percentage - so if you play all mid major teams OOC but chose the eventual mid-major conference champions for your opponents then your SOS will be stronger than a team playing mid level P5 teams because the mid-major conference champions will typically have better overall W/L records than the P5 teams who have conference records of 8/8.
RPI calculations take into account three different W/L records - the Team; the Team's opponents (SOS); and the Team's opponents' opponents. The percentages used for RPI are 25/50/25.
If the all the teams in a conference get their schedule right and play OOC teams that are all just a little worse than them at whatever level they are at, they come out of OOC play with gaudy W/L records, regardless of what their SOS (the opponents W/L record) ends up being, and those gaudy W/L records then assist all their conference mates to the tune of 50% of their RPI calculation plus it also adds to the 25% of the opponents' opponents calculations for the rest of their games:
As an example:
USC went 12-0 in their OOC - no idea what their opponents did and it isn't that important for the rest of this because USC isn't getting into any post season play - they are 6-12 in conference.
When Oregon State now plays USC, their SOS goes up because while USC is a bad team, their record is 18-12 (60% winning) so to the calculation it looks like a 'good' team.
Oregon States RPI calculation also goes up because they get another win (25% RPI weighting) plus USCs W/L record is 18/12 (50% RPI weighting) and the fact that USCs OOC opponents may have losing records is only a 25% RPI weighting hit. And if USC chose carefully in OOC teams that were just worse than them, that 25% hit might actually not be a hit at all.
Where it really gets interesting is now Oregon State plays Arizona State and they both have good winning percentages because they have played mediocre OOC schedules and Beaten up on conference mates like USC. They both get a SOS bump because they play each other. And the RPI calculation:
Oregon State's RPI is 25% for their W/L record which has benefitted by playing USC, 50% is Arizona State's W/L record which has benefitted from playing USC, and the final 25% is benefitted by USC having compiled that 12-0 OOC record.
The perfect line-up of OOC opponents in terms of both SOS and conference RPI calculations would read something like:
Albany 24-4
Maine 24-7
FLGC 26-4
Montana State 21-6
UNC Asheville 22-6
UC Riverside 21-7
Long Beach State 21-7
James Madison 22-5
Hofstra 21-7
UTEP 25-2
etc.
Teams that will compile great W/L records in minor conferences.
USC hit two of the above - Albany and Long Beach State, plus Santa Clara 22-7, Gonzaga 18-12, Iona 20-11 in an OOC that included one NAIA school, one P5 school (WV) and the rest mid majors with only two of them with losing records - Southern Utah and UC Irvine just about the perfect conference mate for the rest of the Pac12 to establish a good RPI.