It isn't so much the margin itself - though nine points is very significant - it's more the quality and character of the two teams in question.
Manchester United look like a team who will carry on winning. They won't blow your mind with the way they play but they have a real knack for winning games and clearly trust in Wayne Rooney and Robin van Persie to get the goals to win matches.
However good a team you are, it is all about your best players: how often you can get them involved and how often you can put them in a situation to win you games. Van Persie and Rooney are put in those positions all the time.
I look at Manchester City and I note how David Silva doesn't find himself in those positions to the same extent; even Sergio Aguero hasn't had a good year in front of goal.
His strike on Sunday was exceptional but it was only his ninth of the season. He had a little spell out injured earlier in the season but it was only a couple of weeks. After his debut season I thought he would be fantastic this year but City have failed to get the best out of him.
It's not his fault because he is a fantastic player. The club have failed to give him the platform to show what he is capable of, and when he plays with Edin Dzeko the Bosnian takes up some of the space Aguero would naturally occupy. He almost gets in the way.
That is the difference between the two sides: United's best players affect so many games whereas the same thing does not happen with City. The system is just not designed to get the best out of their stars. It is all very well talking about Gareth Barry and James Milner but these players are not going to win you league titles.
Fergie had a real problem in the summer because he had so many average players, and instead of trying to deal with the problems he had he just went and guaranteed himself a return when he signed Van Persie.
After losing the title last season on goal difference he also wanted someone to give him a guaranteed edge and Van Persie has certainly done that. He has lifted everyone at that club, whereas City totally wasted the summer window by having no strategy of what they want to achieve with the players they signed.
They brought in Scott Sinclair, Jack Rodwell, Matija Nastasic and Javi Garcia. Which one of those is capable of making a difference? They should have signed a player like Van Persie or Falcao. When you are the best, you can't sign five or six players, you have to recruit one or two who take you onto another level.
United are not perfect by any means. In terms of points won and goals scored, this United team will go close to the records set by Fergie's very best. But in almost every game the opposition have been in it: look at the 1-0 win over Fulham on Saturday. They haven't been winning games convincingly, and that is a weakness. They are winning a lot of high-scoring games by narrow scorelines, but ultimately they are winning.
Football evolves all the time and while for a few years it was all about defending in the Premier League, now the pendulum has swung towards attacking. Teams are opening up and defending is suffering and Fergie has recognised that. He realised quickly there would be more opportunities so he signed a player who could exploit that to the full.
City by contrast are uncertain, and that is the worst thing you can be. They lack identity in attack, and that has allowed them to slip so far back behind United.