Lean UX is focused on the experience under design and is less focused on deliverables than traditional UX. It requires a greater level of collaboration with the entire team. The core objective is to focus on obtaining feedback as early as possible so that it can be used to make quick decisions. The nature of Agile development is to work in rapid, iterative cycles and Lean UX mimics these cycles to ensure that data generated can be used in each iteration.
Personifications of some of our users help us remind ourselves that other people use our site/software in radically different ways. Assuming our users behave exactly as we would is the shortest route to average or terrible results.