I would assume that the tracking number has the address details tied to it (I mean, based on support claims with USPS it definitely does), but maybe there's some obscure package type where destination is not distinct and they abuse that in a pool with other sellers, then at the last mile it gets sorted appropriately?
Seems wild that this market even exists considering how USPS flat rate postage exists.
Was your package something heavy or extremely light which makes weight-based shipping more economical?