Which was stupid. Overwatch must have, ironically, had no overwatch whatsoever in the 'making the rule make sense' phase.
"Okay, guys, this game is supposed to represent simultaneous combat through sequential turns. Now let's make a rule that only makes sense in sequential combat!"
Not only that, but overwatch was crap and possibly the second biggest contributor to the unbearable slowness of the game after looking up of everything you ever did every turn because it's never in the one book.
24" of movement on a Land Speeder is not, and never will, represent them moving directly forwards at the maximum speed. It is representative of them following the terrain, avoiding incoming fire, moving into attack positions, and all that. Even superhuman wariors probably can't do that at full speed, and even if they did they'd have a displacement per hour that' much lower as a result.