I reckon we think '2000 was 10 years ago' because we logically know it was a long time ago, and, as we think in base 10, 'ten years' is an easy subconscious shorthand for 'a while ago, but within memory'
A weird amalgamation of our instinctive and logical thinking processes, but then so many things are.
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This "memory measured by significant events" system could also explain why 2016 can feel so *longer* ago, despite being relatively recent.
There were MANY of significant events in 2016. But our experience suggests that such events are typically spread over longer periods
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