Unfortunately it's a much simpler reason. If Revenue grant you a full years credits against, say a couple of months earnings, then you might pay no tax at all in the year. Then, on review, if it turns out that you had other income from whatever source, that was taxable in IRL in the year, then you may owe them tax on that. It's easier to grant credits week by week until returns are sorted and then give you back money if necessary than to be looking for money off you which you may not have expected to have to pay.