[A*O]

The problem is that a cheque may have "cleared" ie, the funds are put into your account which normally takes 2-3 days when in fact it's still a fake and the problem is only spotted sometimes a long time later when the cheque goes through formal clearing at head office.

Effectively the bank "loans" you the money on the assumption the cheque will eventually clear. Meanwhile you can withdraw the "cleared" funds from your account and spend them. The scam takes advantage of this anomaly in the banking system. THEN the fake is spotted and they come after you for a full refund.