SEA and PDX are getting senior on CA side, SFO seems like it started junior because people didn't want to give up their cushy crj trips before they knew what was going on with trips. Many people in SFO are commuters and trips started off with locals and 2 days. DEN is more senior relative to the rest of the company but within Denver it's not, i.e. CA that would be junior on the CRJ switched over to ERJ to get the upgrade and be senior, instead of staying FO longer to hold Denver one day, it would be the same if SLC got 175 flying, senior FOs (company wise) would take it because they wouldn't hold CA or be junior on the CRJ, so it appears it's going senior when really it's people moving back west or taking the upgrade after waiting it out to hold a particular base,

That
May not have made sense but I tried