{Note - I am going to simplify the numbers below since the US tax code is, well, complicated}
Given the following tax rates:
1..12000 10%
12001..24000 15%
24001..36000 20%
36000 and up 25%
Why are payroll taxes taken out as if the amount you were paid that period is how much you will be paid every period through the end of the year?
For example:
Bob makes $24,000 per year, at the end of the year he would owe the government $3000 at the end of the yes (10% * $11,999 + 15% * $11,999 + 20% * $1).
If he is paid monthly, then his employer will deduct $250 per month in taxes.
However, if he quits at the end of his 6th month, he should have only paid $1200, but his employer has deducted $1500 (half the $3000). So Bob is now entitled to $300 to be returned because his employer overdeducted.
Now take a slightly more complicated version:
Tom works two jobs, each of which pay $24,000 per year. He works the full year, and has made $48,000 when he's all done (gross). Tom should have paid $8400 in taxes, but if his employers deduct as described above, he has underpaid his taxes by $2400 (each employer withheld $3000, but that's not enough).
The two-part question:
A) Why are taxes taken out on a pro-rated basis instead of an as due basis?
B) If someone is working two jobs, why can the employers not deduct the "new correct" amount any time a threshold is hit?
It would seem simpler to deduct taxes as they are due, rather than pro-rating throughout the year, ie on the first $12,000 deduct 10%, then on the first dollar of the next $12,000 deduct 15%, and so on.