Here’s my theory to explain why many have the preceptipn that CS lasts longer than CY. The people who buy CS usually buy it from supermarkets and those who buy CY get it at grocery stores. The drivers who deliver to grocery stores have many smaller deliveries and being that they have less time per delivery (sometimes delivery is before the store is even opened) the milk may not be under constant refrigeration. Supermarkets generally require the driver to actually deliver the milk to their refrigerator and sometimes the shelves are stocked by manufacturer’s employees and not store employees.
Grocery stores and supermarkets have completely different business models and my experiance with kosher supermarkets is that they are often run like supergroceries