It’s not married logic; it’s combinability rules.

You already have a broken fare, because it’s giving you a K to SFO and a W from there. So, either (a) the HKG-SFO K fare only allows end-on-end combinations with W fares or higher, or (b) none of the SFO-PHX fares below W allow end-on-end combinations with an HKG-SFO K fare (or, likely, any HKG-SFO fare at all).

You’re already getting a better price than they’ve published for HKG-PHX. You’re just not getting as good of a price as you would by buying two tickets. That’s common, and it’s an intentional pricing strategy that the airlines use to allow them to offer sale prices in some markets without offering them in all markets.