when you are in L imagine that you are in a manual transmission vehicle and you leave the vehicle in first gear and dont ever shift it out. How fast can you really go with the rpms screaming. That is what L is, just first gear. Most likely the transmission has a rev limiter built in so that you don't burn it or the engine up if you put it in L by mistake.
When it is in drive the transmission is allowed to cycle through the rest of the gears, So it will rev higher to get to a better shift point, then shift into second, third and fourth if you have it.