I've added a seat to a volume license. Why is the support period less than 1 year?

We use a fair pricing model that calculates the new support period based on the average expiry date of all parts of a merged license - since a volume license can have only one common expiry date.

For example, if you have a 10-seat license that expires today, and you add 1 seat, the expiry date of the whole license would be in 33 days. The 10-seat license adds 0 days, while the 1-seat license adds 365 days. This is then divided by 11 to give an average expiry date for the merged license.