If you have a payment problem you have 48 hours to change your payment card until they cancel the order. They hold the order for 48 hours
Obvioulsy issues can happen and they might of oversold or mistakes in warehouses causing you to get a cancelled order due to out of stock even if you do change your payment card
This has happened quite a few times with HMV recently especially with the hateful eight. People placing orders and then getting it cancelled days or weeks after whilst they try to source copies
That's what also could happen with zavvi, they might not have the stock by the time you have sorted the payment problem but they are still sourcing copies hense items coming back in stock sometimes after payment problems being resolved but the order not fulfilled/ cancelled