Good Guess. It IS a speed thing.
In some special cases, it may also a coaching/training thing. If it is done the other way around (WR suits up as RB), they gain a little weight, lose a little speed, so that is definitely a coaching/training thing.
Has anyone else noticed that RB coaches and TE coaches train the exact same skills? Any coach's RB position score will always be exactly the same as his TE position score.