Couldn't this just be that they keep some amount in cold storage that's not super easily accessible?
When you put in a sell order through the normal "retail" interface you're trading with Coinbase itself. Those coins can stay offline in cold storage, as they're just moving numbers around in their database, off-chain. Only withdrawals and periodic internal rebalancing operations (making up for an imbalance between buys & sells) strictly require online access to the keys, and those only affect a small part of the total.