Did you check individual flight bucket availability or married segment availability? Adding segment will change the fare from a PHL-ZRH fare to a PHX-ZRH fare (fares are not priced hop-by-hop). There are other things that can come into play here like advance purchase requirements (just because there is bucket availability in lower fare classes doesn't mean there are cheaper fares that can be purchased in those classes due advance purchase requirements for them). Without further info on dates of flights, etc., there's not much to go on here.