Your Bitcoin balance, as it's shown in Coinbase's UI, is very different from your Bitcoin balance as it's understood by everyone else on the public blockchain; and that too only temporarily. There is nothing that Coinbase can do to permanently alter that supply of Bitcoin. That is what we're talking about.

In contrast, the permanent US Dollar supply can and is permanently altered due to the monetary policy which governs it.

That is, can we add up all of the Bitcoin listed in the customer ledgers of all of the exchanges, or is that information (the total, not the individual entries) not available to auditors?