As I understand it (and I could be wrong), next year's budget uses last years budget as a baseline. If you spent $20K less on school books for example, you'd probably get $20K less for books next year. If instead you spent the same, but got 20% more books for the same money, that might work.

There is also this practice in organizations where money is allocated in "pots"[1]. For example, there is probably a pot for hardware/computers and a different pot for software. If you save them money on hardware, they don't automatically get to spend the savings on software.

(I thought you were unfairly down-voted so I up-voted you.)

[1] The correct term escapes me at the moment.