Outer tracks can be packed with more information.

www.storagereview.com has more information on hard drives that anyone would want. They're the best site in my experience, but they are sometimes wrong. They say in the reference guide that a 8MB buffer doesn't make a big difference- in practice it makes a 30% difference. They also say that RAID doesn't boost performance much (10%), but their own tests with better RAID controllers show much better performance.