The points gap isn't relevant because United had such a poor start to the season that they're always playing catchup. They won 6 or 7 on the trot and remain in 6th despite Arsenal and City dropping plenty of points. Now they've failed to win 2 in a row they could be 6 points behind us again.
When you start that poorly, you need to win a large majority of your remaining games and hope two of Spurs, City, Arsenal or Liverpool don't have a good second half to the season in order for United to finish top 4.
I'm not saying it's impossible. They're just relying too heavily on other teams right now and with Europe and domestic cup competitions returning I wholly expect them to go back to struggling to win games.