It's just simple math really. If a year is said to be the end of a century, say 1999, you only have to divide the year by 100 and the quotient should be a whole number. You don't get that in 1999, you get that in 2000.
It's like buying a 10 kg of rice. You don't pay for a 10kg of rice which tips 9kg in the weighing scale. Lugi ka kung ganun.
That is why I don't understand (and pisses me off) why some people and online publications are churning out their 10 best of this and that. Unless the title says 'Best of the 2000s', that is just unforgivable.