Because it leaves earlier and gets in later than the published options for this route. There are lots of routes where there are alternative options that are legal, but they aren't "published" options unless they either get in earlier or leave later than other options, if it doesn't do either of these, they don't publish it as an option. WN's schedule would become ridiculously cumbersome if they published every single legal option, though it would be nice if the website could identify these on its own in cases where a flight is sold out.