This appears to be a bug in Outlook 2013. In my Inbox, if I do View Settings... then click Sort... and sort by Importance (descending) and then by Flag status (descending) and then by Received (descending), the result shows high importance items at
top, followed by flagged items as appropriate. But following those, the items with regular importance and no flags are not in proper descending received date order. I first have a block of items from Nov2016 back to the current date in 2016, THEN
followed by items from the current date this year descending into the past. Not good.
Turns out that even though the misplaced set of emails from 2016 are not flagged, if i assign a flag to them and then clear the flag, then they go into the proper order by descending date. so, there are apparently unflagged items and then there are
unflagged items. weird.