Why are bonds from different months worth exactly the same amount?

Your Savings Bond Calculator shows that my EE bonds from June and July 1990 have earned exactly the same amount of interest. How can that be? Have they stopped earning interest?

Tom's response

For Savings Bonds issued before May 1997, interest is added to the value of the bond every six months, beginning with the month of issue.

This means that bonds of this vintage issued in June and July will always be worth the same thing except in June and December, when the June bond will be worth more because of an additional interest payment.