Exactly how do you suppose “purchase orders and deliveries” can be “reviewed and compared against inventory” in a restaurant? Is the mashgiach supposed to go into the fridge every hour to count the chickens, calculate how many have been used since he last checked, and see if there’s an extra one or two that shouldn’t have been there?!
Remember that this case is supposedly about an owner smuggling in *small quantities* of treif, on many occasions over a period of months. He walks in with a bag containing two chickens, perhaps already cut up, and when nobody’s looking he dumps them into the bucket or whatever where the chickens currently being used are lying. How is the mashgiach supposed to detect that?