Why do i owe back some of my tax credit? Please help.

I have insurance through the marketplace and receive Advanced Premium Tax Credit based on my income. I always put what my projected income will be, and every year its the same, if not lower, than what I put when I applied, but every year I owe back some of my tax credit. This year I owe back $421, last year it was like $127. I have no idea why this is happening and it's extremely frustrating as someone who doesn't have an extra $421 laying around. I was under the impression that as long as you don't make more than you say you will when you applied for insurance, then you won't owe any back, and yet I always do. Can anyone explain to me why this may keep happening and if there's something I can do to fix it?