The dreaded hunger notion comes into it. Often derided but in a game between evenly matched teams that will or drive (or more importantly sheer refusal to lose, a la ourselves in 2011) can make the difference.
Kerry have that determination to beat us or more importantly not lose again to us. That counts, if the game is close enough.
Similar with Mayo. I firmly believe this Dublin team is and has been comfortably better than them over this past period but that Mayo's sheer strength of character/ resilience/ will to win/ refusal to lose/ hunger/ whatever has helped to keep them competitve. (That's not to deny the obvious abundance of skill they have but it is commonly accepted they are/ were lacking in key areas compared to other teams.)