Why is there only the option of a 1 year or 3 year USDA licensed vaccine duration? Why not a 2 year?

USDA licensed vaccines are only licensed for either 1 year  or 3 years. However, depending on the local/state regulatory authority, they may only allow a vaccination expiration duration of 2 years. Therefore, the expiration date of the vaccination would be set to 2 years, even though the manufacturer's label claim is for 3 years.