has to do with probability of resulting points, involves both sides--meaning you get zero if you punt and what that other team could end of scoring. (they have a higher chance of scoring if they get the ball on the 40 as opposed to the 15) But, if you go and make it, you have a greater than 50% chance of points. (I don't have those numbers in front of me, and I'm not gonna take the time to pull them up, but many teams have them, whether the coach uses them or not.) Anyway, go for it a thousand times on 4th and 2 on the other teams 40, and YOU WILL end up with more points because of that than how many extra points you give up because sometimes you don't get the first down.
A much simpler way to look at would be look at what percentage of Coker's carries did he fail to gain 2 yards? That's not nearly as good of a question, but gives you some idea.