It is said that if you forecast the weather for the next day based on today's weather, you will be correct 70% of the time. This concept is used to determine how much work for which the developers can sign up in any given iteration. The total number of points for which they are allowed to sign up is only as many as were completed in the previous iteration.
This allows the developers to recover after a difficult iteration, and to deal with issues such as vacations and illness. If there is still time left in an iteration after all of the Stories and cleanup tasks have been completed, the developers ask the Customer to provide more Stories in order to fill the time.
The Project Velocity can be expected to fluctuate slightly over time. If there is a substantial change for more than one iteration, a Release Planning meeting should be held in order to re-estimate the remaining Stories and to and renegotiate the release plan.