I was wondering about that too. I took a closer look at what changes. When you increase a couple thousand in income the amount of tax credits and deductions curtail a noticeable amount. And if you are getting your health insurance with subsidies that couple thousand puts you further away from what you estimated when you applied for that subsidy. A couple thousand difference can give the illusion of a 350+ dollar difference easily with just the healthcare insurance subsidy issue. I imagine the further under 30,000 you are the more of an effect a couple thousand has.