in short.
orphans occur. meaning if an exchange accepted a 'payment' with one confirmation (you called it a validation).. then if that transaction was on a block that initially confirmed. an exchange would treat it as payment accepted. give balance to a customer and then the customer would sell the coin and withdraw it. but imagine then, that block is orphaned..
the transaction is then added to a different block and confirms so some exchanges would give customers new balance again.
most of this is just bad coding of the exchange. but the simple solution for exchanges is to just request X confirmations that are above the chances of orphans. thus avoiding exchange balance double spending
orphans happen. and the average is 1%-2% chance per day.
https://blockchain.info/charts/n-orphaned-blocksso accepting zero or 1 confirm has upto 2% chance of being double spent by badly/lazy coded exchanges.
orphans can also happen for other reasons so some exchanges dont want to run the risk of the other reasons.
its also worth noting that orphans can also happen on differing algo's too. so dont be so sure that swapping to POS will solve the confirmation waiting time of exchanges, as exchanges would in some cases still prefer some time limit to reduce risk.
however its much easier to get exchanges to refine their code to add more checks and techniques to limit their double spend risks to make it easier to offer deposits with less confirmations needed.
however just for safety i still think upto 6 confirms can be deemed safe for a majority of transactions of large value, due to some of the risks of orphans in the past. (5 block orphan on october nineth 2015). though things like this are a rare occurance so some leanience is given depending on value being handled vs risk
