>>2568527Because better lenses have better AF systems inside, allowing them to be controlled and to communicate more quickly and more accurately than lenses on the lower end.
I don't know the engineering behind it. I know that when I started with my T3i, I had shit lenses, and hated my AF system, then, following /p/'s advice, I got L glass in stead, and my AF was fast, reliable, and consistent.
