I've been looking at a Craft site for a friend which suddenly started reported incorrect numbers for sales (they're using Craft Commerce). They noticed that one day they had an item that had 62 sales, then when they went back to the reports the following day the sales had dropped to 28. Exporting the database, templates, and modules (the reports from from an externally developed module) and setting up a local host on different machines, they report correctly.
I'm quickly running out of ideas why the numbers are different. The site is hosted on a Linux box, the local sites have been tested on Mac and Windows (both running MAMP and Nitro).