First, nearly everyone is picking Utah to represent the South (PAC12 media voted 33 votes for Utah v. 2 for USC). The North is evenly split between UW (17) and Oregon (17), with WSU (1) getting a vote, despite actually finishing 4th in the preseason poll.

For the top 25 you get to rank all the teams however you want. So if you're a voter and you think UW is the best team, Utah is 2nd, and Oregon is 3rd, you can put them in that order. If you think Utah is the best, Oregon is 2nd, UW is 3rd, you can put them in that order. So regardless of which North team you think is the best, you can put the #2 North team ahead of Utah if you want.

That's not true when you're asked to pick the conference winner. When that happens, you have to pick one team. So if you're sure the North representative will beat Utah (who you presumably picked as the South representative), then you're going to pick Oregon (11 votes) or Washington (9 votes) or even WSU (1 vote, that guy loves WSU). But if you think the South representative could win, that's almost certainly going to be Utah (12 votes) so you're going to vote for Utah, although both of the voters who think USC makes the game also voted them to win it.

So 33 media members picked Utah to make the game, only 12 of those picked them to win it. The same is not true for Oregon or Washington, where they had 17 members each picking them to make the game, but 11 picking Oregon and 9 picking Washington to win it. If Oregon or Washington were the overwhelming North favorite, like Utah is in the South, then they'd have also been the overwhelming favorite to win it, with something like 20 votes.

Hope that helps.