When someone signs up for a program, we tell Google how much we want to spend in that month based off of the program the Agent signed up for. With any Pay Per Click campaign, you also have the option to put a "maximum" on how much you are willing to bid for a click. To best optimize campaigns, we do this and we start off low. This isn't to hinder the campaigns in any way but to try and make sure we get our Agents the most bang for their buck.
A good analogy here would be giving someone $100 to go buy steaks VS giving that same person $100 to go buy steak but telling them not to spend more than $10 per steak. When you put a limit on the cost per steak, you can guarantee they aren't going to buy 1 steak for $100.
What you can't guarantee is that any particular store is going to have steaks for less than $10. Sometimes they can only find 1 or 2 steaks for $10 and they have $80 left over. That is why there is left over budget. We normally start with a lower bid and slowly work it up to try and get the best cost per lead possible.