Yes wrong. The first decade was either 9 or 10 years (depending on whether you want to be weird and include 1 BC), every decade after that was 10 years starting

Yes wrong. The first decade was either 9 or 10 years (depending on whether you want to be weird and include 1 BC), every decade after that was 10 years starting at 'X0 and ending at 'X9. This is how the common usage defines the term. A lot of concepts in daily human lives get fuzzy near the edges. That's not a failing on humans, that's how language works. This isn't scientific or perfectly rational, yes, but neither are humans.