The 3 in a 4 2 3 1 are positioned naturally further up. The three were Martial, Sanchez and Lingard, none of whom are really midfielders in my eyes. They are all players who attack/counter, not players who can help control the game. Neither the system nor the personnel were set up for keeping the ball and I feel like we try the counterattacking setup too often. We also should have changed it as soon as we went behind because you can't counterattack against a team that is only trying to control the game rather than chase it. An extra proper midfielder should mean better control of possession.
Spurs are quite different as Eriksen is like Pogba as a player and in any case they generally line up with Dier or Wanyama at the base, Eriksen to the right and Dembele to the left. I think we can all agree that Dier/Dembele/Eriksen is a proper midfield three and that Matic/Pogba/Lingard is not really... Lingard is more similar to Son if you want to compare players with Spurs, Pogba to Eriksen and we frankly don't have a player like Dembele. Even when Pogba attempts to play that role we then lack the playmaker type just ahead of him.
And all of that is still leaving Alli and Kane out of the discussion who both do quite a bit of defensive work. Lamela and Son sometimes too. It is actually unlike Mourinho to pick so many attacking players. It's not working. Lingard does not have enough freedom, for a start.
In the 4-2-3-1, the '3' play as attacking midfielders when with the ball and 'defending' midfielders when without the ball. Their positionings are incredibly flexible first of all because they are expected to interchange position among themselves (hence we have seen Martial switch wings in the last few games) and secondly because their positioning, relative to their team mates, change depending on who has the ball. If the opposition has the ball, there is an onus on these three players to fall back deeper into the midfield and help out defensively, particularly those playing in the wide roles who are supposed to also help their full backs. At this time, the formation should look more like a 4-4-2 or 4-4-1-1 o even a 4-5-1. Among the three, there is usually one central creative force, playing behind the striker (I have seen Jose deploy either Alexis or Lingard there).
So clearly, a central idea of the 4-2-3-1 is to avoid being outnumbered in the middle.
As for the 'double pivot' or '2', one is often asked to focus on breaking up play while the other often has a freer, roaming role playing as a holding midfielder or a deep lying playmaker. Whoever plays that role often has to possess impressive vision, good passing range and the ability to dictate tempo. One may totally disagree with him and understandably so, but it is so easy to understand why Jose plays Pogba there regularly especially at the time our midfield got depleted.
When his team is in possession of the ball that second of the '2' has a lot of freedom and is more offensively oriented. He often takes up the playmaking role and occupies that space behind the number 10 (that central player of the '3'. Again, this is why it is wrong for anyone to say that Pogba is tied to defensive work when we play the 4-2-3-1).
In the Spurs game, in my opinion, we did not lose because of our formation. We lost because too many players simply did not come to the party. It was shocking. The blunders, poor decision making, lack of bite, etc.