Employers do witholding the US. Filing at the end of the year is doing a calculation to determine if the withholding is correct and if you owe extra or are due

Employers do witholding the US. Filing at the end of the year is doing a calculation to determine if the withholding is correct and if you owe extra or are due a refund. The factors determining total liability aren't simple enough that paycheck withholding can be entirely accurate.