This is based on how much the rope stretches and doesn't really measure how "strong" a rope is. It's just a measure of force put on the falling weight - again from the first fall done in UIAA testing. The stretchier the rope the less force. Think about braking in a car and how a quick emergency stop feels. Also why we use dynamic rope. You might want a less stretchy rope indoors because of the shorter height, and a slightly stretchier rope on long trad runs where you want as little force on the gear as possible. Honestly there isn't really much point worrying about this though.