its basic leverages right? Hold a weight at arms length then hold the same weight closer to your body. Makes a difference right? The distance from shoulder to elbow for bench and hip to knee in squat does have a significant impact on your ability to move weight. Its not the only factor, but it is significant. With powerlifting, however, what you gain from short arms in bench you lose in deadlift. The Russians advocate experimenting with stances and grips to find the right technique for your specific lever lengths- since two people are different it makes sense that they will lift differently.