The refund check was computed by applying the new lowest tax bracket. The new percent means that anyone who pays any taxes would pay $300 less (e.g, the bracket is 10,000, and the percent went from 5% to 2% (off the wall figures)). Instead of changing all tax forms for this year to make everyone pay $300 less at the end of the year, they left the forms alone, and sent you $300 check. Next year, the forms will include $300, so no more checks are forthcoming.