Earlier quoted context omitted.
And the "total volume" below?
It's the total amount of money passing trough the network. But since bitcoin transactions work by sending "change" back to yourself this value is not that relevant. In most cases 90% of the transaction amount is sent back as change. https://en.bitcoin.it/wiki/Change
If the ratio of 'money spent'/'total volume' is 1/5, does that mean that people are, on average, spending one fifth of their wallet?