There's a structural element to that. By legislation, the department cannot spend more than its authorities. If you are a chief financial officer, a deputy minister or an assistant deputy minister, the last thing you want is to “blow your vote”, as public servants call it, which means spending more than you are budgeted for.
Therefore, there's a natural bias towards asking for slightly more to ensure that you will be able to deliver on your mandate without spending more than what you're authorized to spend.
The system—