So the first decade, 2000 years ago had nine years in it. And each subsequent decade has ten. There problem solved.
The real question is why every ten years do we have to subjected to a bunch of know-it-all's running around telling everyone something they already know. It's all a psychological association with numbers. As someone pointed out earlier 1980 is a year that is associated with the 80s, it'd be foolish to try to include it in a discussion of the 70s.