Yes, unfortunately, the IRS adjusts withholding down every year to prevent "bracket creep".
This is based on the theory, if you make about the same amount of money every year, over time, your tax bracket will move up due to inflation, while you effective tax rate may go down. The higher the rate of inflation, the bigger the difference is when compared to last year.
To remedy this for yourself, you may want to file a new W-4 form every year in January.