40y or 100m times are just snapshots. And really, they measure two very different things.
If you look at Usain Bolt's 100m chart upthread, you notice that he spends the entire first 40 yards accelerating, and right at 35-40 yards, his speed plateaus. The rest of it he's maintaining his top speed.
So a 40 yard dash only really measures your ability to accelerate. The 100m can show your top speed. If you're fast in the 40 but with a lower top speed, you'll lose the 100m but might have great football speed. If you have a slower 40 but higher top speed, you might have more of a fluid running style and won't get caught from behind.
And neither of these have any bearing on your ability to change direction laterally (and with balance), which for a football player is usually much more important than straight-line speed.
I think we spend way too much time as fans obsessing about track times.