Why does my refund/tax-due alternate every year?

My situation with respect to taxes hasn't changed for the past 4 years - single, unmarried, no investments and single income. The amount of my income hasn't changed either. But I always alternate between getting a refund of $x and paying federal taxes of just about $x. It goes back & forth every year - which means, I've received 2 refunds & about to pay my 2nd tax bill. How so? FYI, I use TurboTax to file and end up with standard deduction every year with the EZ form.