Why Will-o-wisp has +4 to hit if they have +9 on Dex?

@RChiesa07 Why Will-o-wisp has +4 to hit if they have +9 on Dex #DandD Dex might be there to capture aerial agility, size to some extent, combat skill not a influenced by it. Monsters in 5e break rule

— Mike Mearls (@mikemearls) March 26, 2016