Sorry, this was a red herring. For clarification, I was doing a roundtrip search instead of one-way which is why you are seeing different pricing. See my followup post about AS favoring more expensive AS fare filings over cheaper partner fare filings. It's listing the cheaper partner fare filings in the ribbon fare listings, but it won't actually let you purchase those cheaper partner fares and is only allowing more expensive AS fare filings to be purchased on flights in some cases (where AS codeshares/fare filings are available on flights).