It's very possible that the factory swapped a part out without the RasPi guys being aware, but that doesn't actually happen that often - usually they will ask to swap it and send a datasheet, but if they buy an incorrect part the customer isn't going to pay for it, so this isn't so common.

First article inspection is pretty important because you're very likely to get 10,000 units identical to the first one - that's how factory lines are designed to run. Possibly their testing was booting from an SD card and checking a screen picture came up, which would have caught the more tricky issues like BGA soldering problems, but wouldn't have helped a network issue.

Factories tend to push hard for testing because if there's a problem with the goods they ship, it comes back to them and it's a PITA. They would like to know, with some certainty, that they are shipping working product.