The note in the pop explination seems to indicate that the free months would be added to your account but not change the price you pay.
NOTE: Free months are added to new accounts when choosing billing cycles of 1 year or more:
So if you choose the two year payment option you would get 31 months for the price of 24.
You should be able to get an official answer by sending the question to sales@westhost.com although they are normally pretty good at keeping an eye on this section of the forum.