Red Pro Warranty

I was checking the warranty status of my Red Pro HDDs(that are installed in my Synology DS), and noticed that, 3 of my 9, 4TB drives, appear to only have 3 Years Warranty. Can anybody explain why this may be the case?
I’m comparing the Warranty Status from the Web Site page, against the “Date” on the HDD Label.

Warranty: 5 year
Western Digital WD Red Pro 2TB, SATA III, 7200RPM, 64MB, NAS HDD for 8 to 16 Bay NAS.

The Normal WD Red versions comes with 3 years warranty.