returns are only from miscalculations by the federal system,

Well, not really. More like incomplete calculations, erring on the side of overpayment because people are generally extra upset if they have to pay in at the end of the year, even if they technically made more money as a result. People are used to and kind of ignore the tax portion of the paycheck they never get, but they feel the taxation of actually making an active payment. So people get happy about the zero interest loan they give to the government because in the moment it feels like "free money"

My employer has to do withholding without knowledge of other potential household income, credits, or deductions, though that last is pretty much given for everyone given how high the standard deduction is now.