The operative word being anniversary. Not the 20th year. For example, one year after you are married is your first anniversary - not your 2nd. I think what the mint is doing is pretty standard.
I understand the point, and I'm not quibbling as I could care less what they call it. But it would be nice to at least see some consistency. The mint released the 10th anniversary set in 1995 - and the 20th anniversary in 2006 ?? I'm confused
And there's the difference. 20th year is different than a 20th anniversary.. Car insurance does it on your car. My 1996 Ford pickup (still getting 20+ mpg!) is considered by my insurer to be 11 years old.
To be fair, cars are slightly different though. You could buy a 2007 model today. It's like time travel.