Think of it as punishing you for not understanding your own budget properly - You set the budget, or at least contributed to setting it. If you asked for money you didn't need then there's a good chance that someone else missed out on funding for something because you said you needed the money. If it turns out that you were wrong, especially if you are regularly wrong then they'll start to think you're padding your budget estimates and cut them back on the assumption that you'll make do with a bit less since you usually don't spend it anyway