When you look at your user reports, do you see any bot-like behaviour?
We've suffered recently from an influx of spam accounts. The accounts always have the same first name and last name and username, so are easy to spot. They tend to read 0 posts but some of them are 16.
Jason
Another thought...
What is the default number of posts to display per page in your community? (when viewing a thread)
Is it possible that some people are changing this in their preferences to a multiple like 20 or 30? If so, with each page load in viewing longer threads, they will show more messages read in the stats as they page through the thread.
You might also consider whether you have lots of short threads (<10 posts) or a mix with more long threads, especially popular / active long threads for the time period you are considering. If you have mostly short threads, a page load might only bring up a few messages.
Just a theory...
I was thinking along the lines of what Mark is thinking. If your default thread view is set to 10 posts per page, then whenever someone views a thread it logs 10 post views. That's why there's a big spike at #10.
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