I recently wondered this myself. The best I could determine from Wikipedia [1] of what's known publicly is that it's seemingly an artifact of when the project began. The theory being that it was allotted a designation before the "Century Series" ended in the 1960s. (Further suggesting that other secret projects for F112-F116 were also allotted designations.)

[1] https://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk#Desig...