The problem stems from the confusion of usage of a measurement of a duration (a decade) versus defining when it started (the 80’s decade.) It also stems from the fact that counting decades from the turn from BC to AD means we skipped a 0th year and 0th decade. If we think of the years as decimal numbers, the first decade AD was not 0.0 to 9.99999 for a total of 10 years, it was 1.0 through 10.999999 for a total of ten years. (Do the math if you don’t believe me 10.99999-1.0 = 9.99999, which rounds to 10 years, a decade.)
This means, if you decide to try to label SPECIFIC decades by counting them (again, starting from one and not from zero), that the first decade started at 1AD and all subsequent decades have started with first year after a multiple of 10. The 2nd decade was 11.0AD to 20.99999AD (21-11 = 10.) The third decade was 21.0AD to 30.9999AD (31-21=10) and so on. The current decade, in this numbering scheme is from 2011.0 through 2020.99999, which means we are still in the 202nd decade until Dec 31st this year (2020.)
None of this abides our humanity. We like to group like with like. So we say that all 10 of the years that start with the same digits are the n’s. So the years in the 20th century that all started with 198 and followed by the digits 0 to 9 we refer to as the 80’s. Since a decade is still just a duration, this is just a different way of accounting for 10 years, because it starts one year earlier than the counting from 1AD. And confusion reigns… another great part of humanity.