That’s the Y2K “lost decade” speaking to you; 1990-1996 was the early 90s, Y2K from 1997 to 2003, and the McBling 00s from 2004 to 2008. I have reason to believe cultural decades started as decades, yet were upended by 1945-65 being The 50s but 1965-69 being the 60s and never been quite on schedule since.
Since the 60s were barely 5 years, there was - apparently - a subtle difference between the early and late 70s, a noticeable difference between the early and late 1980s, and a pronounced difference between 1990, 1994 and 1999. By 2004 a fashion lasted 5 years at best, and 2008 has hardly been a similar year to 2013 or 2019 or 2023.