There is a lot of crap teams in the league and only six top teams, the difference between being top and competing for a top four position is always about how efficient your team is, when it comes to beating these teams.
United in Ferguson's latter years, would always win a couple of big games, draw/lose the rest of the big games but beating almost all of the smaller sides early, and that's how they always ended up in the title chase, and the same thing going on for Chelsea now.