the tax law was found to be invalid by a court, so all taxes paid under the law had to revert to the rate paid under the previous law. However, only taxes paid in the last four years are subject to court rulings like this due to statutes of limitation, which is why the CFTB went back four years, and not zero (everyone paying after the law was found to be invalid would have a 14th amendment suit since the law is invalid), three (see previous) or five (statute of limitations).