Unexpected change in review count

I have periodically noted a sudden rather large increase in the reported count of review card that are due.
Some background: I have not changed the defaults of 200 reviews max or 20 new cards max. I am using Scheduler V3, mostly with Anki 2.1.54 on my laptop, occasionally with the most current iOS version. I do my reviews in the middle of the day. Anki switches to a new date at 4 hours post midnight which IIRC is the default setting and I have never done any review either late in the evening or early in the morning. The date/time on my devices appear to be correct.
Today when I started my review Anki reported 20 new cards due and ~100 review cards due. That number was surprising because most days I see 150-170 review cards due.
I worked through the day’s reviews and may have buried 2-3 cards. I think I edited a couple of cards as well.
After a while Anki displayed the notice that all of the cards that were due had been reviewed but there were ~4 cards due later in the day with the next review in about 7 minutes. I synced Anki and left it alone for perhaps an hour. When I returned Anki showed 72 cards due for review.
I have not done any reviews on my iPhone for several days but I may have synced the iPhone during the day. I usually try to keep it synced with the desktop app even when I don’t use the phone for Anki.
I have noted similar behavior several times in the past, but the unexpected jump in the number of cards due does not occur frequently.
I don’t know how to go about troubleshooting and/or understanding the problem.

This might be due to your scheduling settings, in particular the review steps. A card may be rescheduled the same day, but depending on how much delay there is, you may or may not see it in the same session. By default, IIRC, Anki will show you cards that were scheduled at most 10 minutes from now (that is, if you were supposed to see a card within ten minutes, Anki will show it to you anyways). This means that if reach a step of, say, 30 minutes, then you won’t see it right away, but you will later in the day.

If you find it happens agin, please search for is:buried in the browse screen to see how many cards are buried. Maybe the automatic unburying at the start of the day didn’t happen for some reason?

I understand that, and in my case the “Learn ahead limit” is set for 6 minutes. When Anki reported that I was done for the moment but had ~4 cards due later in the day I expected the number of reviews to be ~4 on my return, not ~70. Have I misunderstood what Anki is reporting re: cards due later in the day?

I guess this might be related, though I don’t know how the number could be so high. I doubt I have ever buried more than about 5 cards during any single day. I do have a fair number of sibling cards in the deck so there are cards that are automatically buried (157 as of this morning) but I don’t know if that might be connected.
In any event I will check on the ‘buried’ count if I see an unexpectedly low number of pending reviews in the future.