I don't know how many times this has to be repeated, but "games in hand" is not the same thing as "matches which were postponed."
United still has to play Spurs, Chelsea, and Liverpool too. We each have to play them once, that's how it works.
If anything, the most accurate way to think about "games in hand" vis-a-vis United specifically are games against teams that are not common opponents for the two squads.
Teams Arsenal has to play that United doesn't: Wolves, Villa, Southampton, Leeds, West Ham, Newcastle.
Teams United has to play that Arsenal doesn't: Man City, Brentford, Norwich.
If we imagined for a moment that both squads got equal points against their common opponents, then Arsenal would need 4 more points out of those six games than United got out of their three games in order to draw even in the table.