Simply put, Like the no follow / do follow example above, Google are expecting links that are of a mixed nature, not all links are created equally, and I choose sites carefully to show natural diversity in the linking pattern, You should NEVER over optimise (or Optimize for my American friends ) for just keywords alone, Google Panda, Hummingbird and Penguin are watching carefully for over optimisation, so we'll keep things natural just to be on the safe side, Eh?