This is most relative in case of e-commerce players like Flipkart, Snapdeal following Managed Market place model. Different suppliers might be located on different geographical locations far away from each other. In that case it becomes a challenge to deliver on time all the goods ordered in same invoice.
Example- Customer sitting in New York orders 3 products 1 Mobile phone, 1 Shirt and 1 watch together under one invoice. But suppliers of these products are located in Los Angeles, San Francisco and Atlanta i.e. 3 different parts of countries. Order will only be considered COTD if all products reach to customer on time. Even if one gets delayed customer service quality will be rated as low and that particular order will not be given status of COTD.