Ill give you an example. If you enter a W-2 with $5000 wages, you might get back all of the federal withholding and qualify for earned income credit. You add another W-2 for $50,000 and not only do you lose the credit, the withholding on the $5000 was probably too low.
Or it can be the other way around. You have a $60,000 W-2, you get a small refund based on the withholding. You then enter another W-2 for $20,000 that may have been severely under-withheld. This is actually common, because the 2nd employer may only tax you as if you are only making $20,000 for the year, when in fact you are at the $80,000 tax bracket.
