Esther 3:7, Question 6. Why does the verse mention that Adar is the twelfth month?

Every now and again, the Jewish calendar requires us to add an extra “leap” month. The Tosefta on Sanhedrin points out that if we were to add a thirteenth month anywhere else in the Jewish calendar in the process of adding this month, Adar would no longer be the twelfth month. That would contradict this Scriptural verse. Therefore, this verse forces Adar to always be the twelfth month.