Sometimes it is because part of the address is missing. Sometimes it is because all or part of the label has been torn off, and it is therefore missing part of the address.
Most commonly, it was at the wrong route or station. There is not such thing as the Carrier taking it out to the route and then noticing that it didn't belong on their route till later. Carriers check the address on each package when they load their vehicles so they know where in their vehicle to place the package for delivery. If the package was missorted to the wrong carrier route, the carrier would scan it as "undeliverable" and then give it to the clerk for resorting. Delivery would be that same day, or worst case scenario, the next day.
Because estimated delivery was two days later, it is most likely that it was sent to the wrong station and scanned as undeliverable (there is currently no scan for "missent"). But your package was making quick progress in the system anyway, and that is why the delivery estimate was moved up.
Undeliverable as addressed has nothing to do with the time. It means it's missing a street name, street number, apartment number or the address is not readable. There are also things that could happen that would give it that scan(clerk throws it to wrong route and the carrier thinks it's a bad number even though it goes to another route). What probably happened is the clerk threw it to the wrong route and the carrier didn't notice until he was always on the street delivering mail. Carriers MUST scan every item no matter what...and there is no option to scan it "wrong route". So he scanned it undeliverable. When he went back to the station he gave it back to the clerk and said it is the wrong route. The clerk then would, in theory, give it to the correct route. It should be delivered the next day although that's not always the case. If they can't find the parcel I would call them and say "Can you please check the return to sender parcel APC. I believe it is being returned when it's not supposed to be". I am willing to bet it's in the return to sender bin just sitting there waiting for the clerks to get to it.
Is this USPS delivery? I've had that happen too, and the real reason is that the package was missorted at the destination post office and sent out with the wrong route carrier. When the carrier realizes that, he/she scans it for resort and redelivery. Their online system, however, show that "undeliverable" status - which is misleading. Give it another day or two.
Not unusual for a package delivery service to miss you once - Maybe wrong side of street. But they usually get it right the second time.
Could just be a data entry mistake somewhere; those happen. If the postmistress is going to talk with the carrier and call you back, then there is really nothing to be done until then.
