From what I can gather you got most of your kills post 20. You didn't get any objectives early meaning you had little to no presence in the game datawise. You got your kills through picks in the jungle and then in teamfights. At which point Ashe is most likely in those fights with you and she probably ran around the map considering how she kept dying to everyone. She also warded a bit more than you. I'm not justifying the MVP score I'm just saying how they probably viewed the game. I honestly thing if you had popped off way earlier in the game you would have gotten a much higher score and therefore MVP. Those with impact earlier in the game who basically set the game up whether it be through kills or objectives tend to get MVP. You got most of your kills and objectives later in the game. You lost the first objective and lost herald to get infernal at 18 min and then eventually your team started to scale and then win.