That duration was measured by a legal clock rather than a natural clock.

Because of that, since 1995, it has been 20 years from filing; prior to that it was 20 years from filing OR 17 years from issue. That enabled "submarine patents", wherein the applicant could intentionally delay consideration of the application to extent the period of market exclusivity.

Design patents have a shorter term, but no one cares how long they last because they are largely worthless, and more like trademarks than actual patents.