The leader boards are rolling 6 months count of the most kudo'ed authors and posts. This allows the opportunity newer members to get on the leader boards and those who were once more active move off. Also, the leader boards are top down from the location. So for example the leader board on the homepage is for the entire Community and the leader board on the Products and Solutions category page is limited to just kudos in that area. If you look at your profiles you will see your total kudo count. Let us know if you have any other questions.