Starting in October, we're introducing "Budget smoothing", another new feature in which the Overture system makes adjustments to the frequency in which your listings are displayed based on the depletion of your specified budget amount. For example, if you have budgeted €100,00 a day, our system will display your listings just enough so that you receive clicks that will total approximately €100,00 for the day. The smoothing feature also regulates the display of your listings so that you receive clicks throughout the day, rather than burning through your budgeted amount in just a few hours.

From Overture Germany (not yet sure when the global rollout will occur. Will likely know soon...)

thanks to AussieWebmaster @ SEW forums