Because the lower wage job will automatically be withheld at a lower rate and when it is added to the other income it will be under withheld. For instance if you have a job where you only make $10,000 for the year then if you withhold at 1 allowance you will have nothing withheld ... but if you add it to $50,000 of other income you need to withhold at zero allowances and that will probably be too little as well.
I find that married couples run into this situation a lot ... if the one spouse in in the 20%+ bracket and the spouse is in the 0% bracket and they each withhold as married & 1 allowance then they will be under withheld & they will owe big time. So the lower wage needs to be withheld at a higher rate which means the need to claim zero allowances and probably have some extra withholding on line 6 to make up for the shortfall. Of course the higher wage could compensate for the lower wage by increasing their withholding.
In the end it is a balancing act for sure ... best to compute how much will be needed to be withheld then look at the PUB 15 tables to find the correct combination that works. <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p15.pdf">https://www.irs.gov/pub/irs-pdf/p15.pdf</a>
