Why the government might owe you money
When you start working mid-year, your employer withholds tax on your salary as if you were earning that amount over a full twelve months. For an incomplete year, you have therefore paid more tax than you actually owe.
The annual tax reconciliation (décompte annuel) is meant for exactly that: recalculating tax on what you actually earned and refunding the difference. It is not automatic, you have to request it.
