Speed

Their centre broke through and outran my fullback and centres. All 3 players have speed of 40, but he accelerated away. He has a speed of 17.
How can that happen?
Bug?
Player bonus / morale much higher than yours perhaps?
Shouldn't be that, my morale is 90+ and better defensive acceleration as well from what I remember. Player vs player, mine was higher in just about every facet.
In your strategy, try to increase Attack speed and Defense speed (not too much or your players will fail their pass / tackle)
Also the speed skill isnt the speed the players is running, its how fast they react in general. Theres another skill [attack acceleration] this is what makes a player run fast once he got the ball. if your speed is 20 and attack accelerate is 40 or higher the player will outrun a player with high speed skill only
Have a look to the help page (in advanced strategy and in player's instructions).

The attack acceleration is a used as factor of player's speed. The player's speed is the base running speed.