Basically say I worked at a job where I didnt make a lot, so thanks to the numbers, and the fact that I want money, I take out only what the govt needs, and they dont need much because im in a low bracket.
Now imagine after 3-4 months of this, where I have a pretty good bit of income...say 8000+, I leave the job for a new one, which is in an entirely different bracket. So when taxes roll around, the income from the first job is lumped in, but relatively speaking the govt goes HAY U IN THIS BRACKET U NOT CHARGED ENUFF FOR MONTHS 1-4 and then I cry and they take from my refund. Thankfully, in my current job I have plenty taken out now that the need for money isnt as dire and im worried about fucking up major in this bracket.