Every year there are always a couple of teams left out who have legitimate gripe and couple that get in that are total head scratchers - the later are often from power 5 conferences who are barely .500, played a tough schedule and maybe finished strong, but on the whole, would not have had much of a complaint if they had been left out.
I'd love to say that it is absolutely the best 68 teams but between the fact it is enough of a subjective measure and politics, I believe that there are some considerations made for things other than being one of the best 68. I am not suggesting AZ will get in...but I would bet anything, that no matter how things turn out in the PAC12- we get 2 teams in.
Right or Wrong, if a blue blood is on the bubble, I bet they get little more discussion. Not necessarily an advantage, but no one is gonna care if Mississippi State .500 and had a miserable year and misses the tourney. UCLA or AZ or Stanford...different story. I think this extends to power 5 conference too. The committee (and NCAA) will want to prop up a failing power conference of there is an reasonably uncontroversial way to do so.
Also, as cynical as it may sound, I think media markets do matter at a certain point. We might literally be looking at only 1 team from California. Lunardi right now has UC Irvine as the only California school. This would be the first tournament of 64 (1985) that does not have at least one of: UCLA, USC, Stanford or Cal. But also no Fresno, SDSU, St Mary's, Pepperdine, San Francisco, Loyola Marymount... usually 2-3 of those make it too. I know the section committee doesn't look at media markets but you can bet someone at the NCAA and CBS and TNT are paying attention. Texas by comparison has 7. Michigan 5, NY 4... This will matter to someone. I believe that if Stanford (unlikely) or USC can finish strong and end up on the bubble and do well in the PAC12 tourney - they would get a bid, maybe play in or something... Cal is not going anywhere except to Weiking Jones professional funeral. AZ and UCLA will have to win the tournament and that seems more like for UCLA than us at this point.
I believe a second PAC12 team will represent one of the lowest barriers to entry but will indeed happen this year.