Why does the W3 not have the exact percentages for Social Security and Medicare?

If you take the Social Security wages on the W3 and multiply by the rate for that year (say 6.2%) or the Medicare wages times 1.45%, you will typically end up with slightly different numbers than those reported on the W3. That is expected because the W3 is simply a total of all W2s added together. When you figure the amount per person (on the W2s) and add them together, the amount will be slightly different than if you add all the wages together first and then take the percentage. Typically the difference is in pennies, not dollars, but the more employees you have the greater the difference may be. There is nothing to be concerned about as both the W2s and W3 are correct.