The only explanation I can think of for this is that there's a threshold
...under which it costs more than the payment to accept it. Say the bank charges the shop 50p per transaction, then anything you buy in the shop for less than 50p loses them money (cost of goods not counted). If they can reject certain transaction sand are not charged for them, then it saves a few pence. But then the time trawling through to choose those transactions would probably cost more than the savings.

Yeah, I've changed my mind. In a sane world, where big companies are big because they make money, this couldn't happen.
(sabre-tooth monkey, Fri 11 Nov 2011, 9:33, 2 replies)

In the old days
before chip and PIN

I was told that big shops such as supermarkets took your card details and signature, assumed everything was OK and then processed all the payments in one go (end of the day or overnight).
The card details were checked realtime to make sure they weren't cancelled but the balance wasn't.
Randomly, the card company would not allow the transaction automatically and they shop would have to ring up to get it authorised (I don't think this is the same as the fraud detection phones calls these days).

Might be bollocks, but if the supermarkets still process all the payments overnight then some might get lost in the system somehow.
( thistle, Fri 11 Nov 2011, 21:59, closed)

Large Shops have what is known as a "Floor Limit" above which things need authorisation. This is, mainly, due to the sheer volume of transactions. So the card details are only checked against a hot cards file, and left to sit to be authorised later that night. This can be quite a small limit, so it stands to reason, that if they are charged 50p for a debit card transaction, and you've spent
( Alex Save a Tree, Eat a Beaver!, Sun 13 Nov 2011, 2:06, closed)