Stats show ancient review occurrences for 'collection', although root deck reviews are shown correctly

First of all, I resolved the issue, so no need for help, but I’m left wondering.

How can a collection have different review history than all the decks in that collection?
i.e. on Stats page, ‘all history’ for the ‘Default’ deck (under which all my other decks reside) shows correct review history, but choosing ‘collection’ shows some odd reviews from ~10 years ago, when I didn’t use Anki.

As ALWAYS, I suspect this is the voodoo of importing dirty shared decks from the internet, practise I’ve since come to shun with vigour.

I resolved the issue by exporting all my sub-decks one by one, deleting the local user, creating a new user, and importing the sub-decks back in.

When viewing stats for the entire collection, it includes reviews associated with cards that have since been deleted.