It's optimizing for different environments and opponents, the speed runs only have to deal with the AI not human players and take such crazy paths the AI isn't around to really mess with them so they don't have to worry about reaccelerating. In versus it's much less likely you'll be able to avoid all the other players so you need to be able to get back up to speed after being hit. A common strategy in the online vs mode is actually back running to collect better items then cashing them in rapidly to win.