There are no '06s, so that solves that.
The vehicle is just a bit too new. Used car values are based on what similar models with similar options are selling for in a particular area. It takes a while to compile a database large enough to provide an accurate guesstimation of value.
Kelly, who compiles the Blue Book, typically will have the value of a base vehicle with no options, and then determine the value of individual options based on how desirable those options are. Considerations are made for option groups. The final value is then tweaked based on condition, miles, and location.
For an idea, a used '07 would probably be only worth about 35% of the original sticker price, at best.
Scott-SVT-
Both the Explorer Sport and Sport Trac have independent price structures of the Explorer. The Blue Book is pretty good at covering all bases. Different cabs and base models (FX4, King Ranch, etc) will have different values.