Perhaps it’s figured on the the worst case senaro, if you weren’t elegible for any deductions at all this would be how much you owe.
Below this point is my personal opinion…
Or perhaps you could look at it this way. If you personally (instead of Uncle Sam) were incharge of gathering withholdings for workers. Would you:
1 - withhold the exact amount.
2 - withhold more then the exact amount.
3 - withhold less then the exact amount.
Now factor in that you are (in this example) Uncle Sam.
If you withhold #1 then some will have to pay a little and some would get a refund (as you would not hit it exactly for everyone.
If you withhold #2 then you get use of this extra money intrest free until you have to issue a refund, have the benefit of people thinking you are good in giving a people a check once a year for a significant amount (having set it up so they never really see the gov’t taking the money throughout the year, people just figure they are getting paid the net, not gross).
If you withhold #3 they you have to depend on people to save enough to pay the taxes and people will get pretty pissed if they really knew how much they are paying to Uncle Sam, this will make them realize that not only are they paying but have been paying over the year.