People send (voluntarily or by mistake) coins to addresses nobody controls. (https://blockchain.info/address/1BitcoinEaterAddressDontSendf59kuE)
Or have stored their coins in an address which the private key has been lost.
This are the ones that are lost. Computers that mined long ago where they get a lot of bitcoins in a low price. like 10,000 bitcoins for just how many dollars. Those computers that cannot be opened now have some bitcoins in there which are the lost ones. It is like a locked treasure box with golds from a sunken ship. It needs to be opened again with a key to go back to the circulation.