This isn't really related, as I understand it.
The Norwegian VOEC system[1] is modeled on the EU IOSS system[2].
As with normal import declarations, missing documentation means you cannot claim any exceptions etc. Thus if the store forgot to register the VOEC number when they shipped the item, the one filing the declaration, say the postal service, has to assume VAT hasn't been paid.
Now, if this had been a normal customs declaration, one could simply submit a corrected declaration with the correct documentation later. However, since such low-value goods aren't declared using a normal customs declaration, one cannot do this.
They also don't want spend their resources cleaning up someone else's mess, thus they leave it up to the shop that made the error to fix the problem by reimbursing you the VAT they collected.
And yes, as a customer this sucks, as the shop often will just shrug and claim they don't know or they did the right thing.
[1]: https://www.skatteetaten.no/en/person/duties/purchases-from-...
[2]: https://vat-one-stop-shop.ec.europa.eu/index_en