It's not rare. There are plenty of examples like that, but you have made a theory based on too little information. Others are also adding 2 + 2 and coming up with "answers" that don't fit every case.
1. For a while, KCC were asking for documents with some planning method (possibly by DS260 processed date) - so that not all cases received requests for documents.
2. In October KCC greatly increased the scope of the document requests, even to the point that they asked cases from "all" case number ranges, whether people had submitted the DS260 or not. This probably created some backlog by about November/December.
3. Some embassies have document concerns and so will ask for some additional processing on documents, but not all embassies are like that. Moscow for example used to place cases on AP because they had special fraud concerns and had implemented special procedures prior to the introduction of the documents procedure. It is likely that they have transferred their special procedures to the front end, based on documents.
4. Because of the wide net cast in point 2, and the additional attention required for some embassies as described in point 3, SOME people are experiencing delays, sometimes along with most applicants at certain embassies. There are other people that are receiving documents satisfaction emails within days of submission. That continues right up to now.
5. Some people are being told they cannot discuss the cases until the number is current. That does not happen in every case either.
6. Some other people report they have not been asked for documents yet - sometimes with case numbers that are current or soon to be current. In some cases this will certainly be missed emails - but it might be more than that - we can't be certain.
So - it would be incorrect to assume there is a "one size fits all" statement here. There are certainly delays and backlogs for some people, others are not experiencing such delays. In documents processing, just like DS260 processing, the processing time can vary case to case - sometimes based on specific embassies, but also there could be other factors (name check matches and so on).