Could this be while newsagents make returns? Actually what 'does' happen to returns?
I’ve no idea these days. In olden times, it depended on the publisher. If they’d pay to have things sent back, everything would be returned. If not, stores would tear off the covers from unsold issues and bin the rest.
the systemic problems there seems to be in getting large-envelope-printed-stuff through the post from the UK to Norway
UK to UK is also still very bad. It seems to specifically be a problem with ‘large letter’ post, for whatever reason. Parcels are rattling through the system OK as are normal letters. But mags are still showing up pretty randomly here. (Although they do at least always show up—so far.)