Why do people think centuries end one year before they do?

[Flilix]

barneystinson69 said:

RolStoppable said:
Hold on a second. How can the first part be correct? When a new century starts in 2001, then a new millenium also starts in 2001. After all, a millenium is exactly ten centuries, so it has to align with them.

https://en.wikipedia.org/wiki/Millennium

Ok, so now I'm confused. Some say it should be celebrating in 2000, others say 2001.

The millenium started on the 1st of January 2001, but I guess '2000' sounded better for the celebrations.