The Orbitz fares include their $5 fee, right? So most of those would be effectively the same on the other airline's site. Also many legacies' fare sales expire May 10: just a coincidence that that's the end of WN's available schedule?
Not claiming a lot of knowledge here, just observing: WN sets many baseline low fares, although sometimes other competition keeps them down. You might find that WN's low fare buckets sell out sooner, though.