Consider how strong the teams are in the top 4 and the quality and experience of each of their managers, and the amount spent on the team that finished 11 points behind in 6th.
Had it not been for injuries to Partey, Tierney and Tomiyasu, they probably would have made top 4.
Arsenal are progressing year on year under Arteta. He has assembled a good, young squad and they are pay less than half what United do on salaries.
They may still not make top 5 this year, but I do think they will earn more points than they did last year. Which is what Arteta has done the last 3 seasons (56 > 61 > 69).