The number of tubs of cat food I've received is different from what's on the order confirmation. Why?

Currently, our system calculates the number of tubs based on 500g, which is the weight of the dog food tubs. Our cat food tubs hold 250g. Therefore,  the number on the order confirmation is incorrect. For example, if the order confirmation says you have will 8 tubs, you will in fact receive 16 x 250g tubs of food.