I may not have phrased that well, but realistically, both the arms and legs have developed muscle memory and fast-twitch muscle fiber, but the arms also have significantly more sustained force muscle fiber*. It's a result of training the arms for both speed and power while training the legs almost exclusively for speed. Long-term recreational and semi professinal boxers tend to be in remarkable shape, long-term damage from repeated head trauma not-withstanding, but the sharp contrast between their arms and legs just looks a little funny to me.
For the record, I don't mean any disrespect if you're reading this and you're a boxer.
*I should really learn the names for those two different kinds of muscle fiber.