The query is:

  SELECT item_id, COUNT(*) AS c
  FROM orders
  GROUP BY item_id
  ORDER BY c DESC
  LIMIT 10;

They are getting the top 10 most frequent values of item_id. It's possible for one item_id to be the 11th most popular item across every partition and not the 10th. Then when you sum the counts across all partitions, it becomes the 10th most common.