Coinbase allows BTC to become spendable after zero confirmations as long as they are in sync with the BTC blockchain, which they usually lag behind a bit.
The reason Coinbase lags behind the bitcoin blockchain is because their internal node is generally lagging behind the blockchain about 2-4 blocks; sometimes less, sometimes much more. I don't know why, but I imagine it helps reduce fraud with instant buys and maybe the system they use to sync their services with the blockchain gets bogged down under heavy loads..
Coinbase does have a network page (https://coinbase.com/network) that allows one to check the status of the last block seen on their side. Therefore, if it shows "Last block created 57 minutes ago," one can safely assume they are about 5-6 blocks behind the btc blockchain and will take some time to catch up.
It can be quite annoying and cause a purchase to be delayed or cancelled, but by watching the network page, one can make more informed decisions about when to send.