I have yet to see any response on this issue from Google, which is
fairly maddening.
For me, the problem manifested as an old headline post, "plus X more."
The X would vary from 5 to 13, none of which had been edited, deleted,
or were new. One option which seems to help the issue is the "nuclear"
option of resyncing the feed. I held off doing this at first, as the
"nuclear option" text was intimidating and not well-explained.
However, having done that, the problem has abated (for now.) In the
past, this option has worked for a week or so.