Why is “speed” a gray skill for DC players?

[Toxcatl]

So from reading in forum most people recommend training only white skills and that’s what I have been doing lately but had a question.
DC players their skill “speed” is gray, so my players are low on that skill.
But if I want to play offside trap, my knowledge is that my defenders should be faster than my opponent attackers so why is “speed” a gray skill?
Anyone can share some more insight on this?

Individual skills don't seem to matter. Don't try to find logic here, this game is not a football simulator.

Also,speed is not important for offside trap in real football, is it? Your opponent have to be offside in the moment of pass. When the pass is not made, your player don't need to run fast; and when it's made, it's too late to run anywhere to create an offside. I'd say positioning is way more important in this case.