As described by Ashi-Certified Inspector Britt B., this discovery is an instance of date discrepancy. The water heater's actual installation date is 12/2020, which means it was installed in December 2020. However, the manufacturing date is 9/2021, indicating that the water heater was manufactured in September 2021.
This means that there is a discrepancy between the installation date and the manufacturing date. It is unusual for a water heater to be installed before it was manufactured. It's essential to verify the accuracy of these dates and ensure that the installation was done correctly and safely, regardless of the manufacturing date. Consulting the installer or a qualified professional can help clarify any inconsistencies and ensure the water heater is functioning as intended.