Because they are selling them individually rather than in boxes there is no guarantee - yes, to us it makes sense but they may simply open boxes as click and collect requests happen. So if someone ordered 6 and the next person 4 then you come and order 24, the warehouse staff may simply send a 24 pack or they may have to send out 14 from the open box and 10 from a new open. With a supermarket like Asda who knows. *eye roll at their logic sometimes*
If your local store doesnt carry the product you probably have a better bet of getting a complete box as if the store has some in stock they may simply be sent enough to make up your order rather than a complete set.
But it is click and collect so if you dont wish the product when you see it you could always ask for a refund.