If I understand correctly, the words September, October, November and December all come from French counterpart Septembre, Octobre, Novembre and Decembre, and ultimately from Latin septem, octo, novem and decem with a suffix -bre. Therefore I suppose their original meaning were from "7th month of a year" to "10th month of a year".
Apparently those words stand for "9th month" to "12th month" nowadays, so why and how did this happen?