There is no way to know what happened to them. They may have gotten "lost in the mail" in a selective way. For example, if more ended up in Democrat mailboxes v. Republican mailboxes that could substantially change the election outcome.

Even if everything is honest and above board on all ends, and no one within the chain of custody had any nefarious designs, it shows that the outcome of the election could have been compromised on accident. Who knows what might have happened on purpose.