Before all this price pumping happened like $360 per bitcoin there are already lost coins and it keeps on adding up whenever a user is creating a wallet then joining faucet websites.
I am one of that examples.
I have some wallets that are forgotten and I know there are satoshis left there.
I also knew that there were a lot of us doing the same faucet work since the deposit amount is being consumed everyday.
Imagine how many satoshis are out there in different wallets which was not withdrawn.
That could be a lot also when summed up.
And in some cases, if people create an account to an exchange, what they are doing is to withdraw their funds if it becomes high enough for them to consider profit. But the reality is that, not all exact amount of bitcoin could be withdrawn meaning to say that if a sure will change his platform or choose better exchanges, there will still be a portion of bitcoin left to his account and if we sum up all of the bitcoins from all kinds of users like this, then a huge portion of missing bitcoin could come up.