We only finished 6th because Brighton did us a favor in the dying minutes of the season. Too many variables, swap our playing staff with Ajax's and I don't think they finish 6th, replace United with Ajax in the league last season and it might be different. They are a well run club, a well coached team, they play good football, they score a ton, don't concede much. They beat Dortmund and Sporting Lisbon twice, those teams are probably better than your average mid table english team.
We finished 6th because we did better than the team that finished 7th over the season. Of which them losing a game to Brighton was part of that. Otherwise perhaps Brighton were doing West Ham a favour by beating us a couple of weeks earlier?
They beat Dortmund and Sporting and then got eliminated against Benfica. United also won their CL group.