There is no real definitive answer to that. What happens is, is certain defects are discussed upon other iPhone owners of what they are experiencing with their new iPhones. For example, A yellow display. So naturally, other iPhone owners start asking which week was other iPhones manufactured during which week that others are experiencing with yellow displays. Then those iPhone owners speculate that "Week 33" might be a bad week for the iPhones that were experiencing yellow displays. Again, there is no way proving this. But they use it as a reference to determine where their device falls in line when it was manufactured on what week.