Question on the 17.6% lost BTC

Hello,

I want to understand what are these 17.6% lost BTC (is that number even verifiable?)

My current understanding is that since bitcoin mining started back in January 2009, many people must have had mined quite a few and then changed computers, some old computer got thrown away, some user died with their wallet, some other mined btc (while they were 1$ or so) and then forgot, non-technical people might have lost their password, other had their storage destroyed, lost ...

How far am I from the truth?