Hello Mark,
Thank you for your question about the Completed list. I believe this may be a bug, but it looks like the "Completed" tally is including items in your Archive folder.
The Completed tab count should only reflect items from the last 30 days, after which they move to the Archive.
Does this explanation match what you are seeing? If not, please let me know so I can add a follow-up task.
bidslammer