Married to a former government employee and I am told that when an agency has a budget they have to use up all the money in the budget or it goes away, they can even be given a smaller budget the next year because they didn't need as much the year before.
Worse, iirc, that money you don't spend doesn't go back to the big pot, but gets moved to the budgets of programs and offices that weren't as careful as you and went over budget...