Please take a look at this forum thread, which discusses a similar issue, I believe you might find it useful. Generally the problem might arise if you have different dates set for the same content item, For example, if you have a news item, due to the ContentLifeCycle implementation that item has a Master state (the one you see in the grid when going to Content->News), a Live state (the one you see on the frontend in the News widget) and also a Temp state (when you open an item for editing). Besides that, an item has DateCreated and PublicationDate. In case there' s some discrepancy between them, the News widget will display one date for the same item on the forntend and a different one in the backend (since DateCreated is used for backend and PublicationDate for frontend). This can also cause the archive widget to display unexpected results.
I see that you have specified Sitefinity 4.0 product version. We used to have some problems with the archive widget functionality, displaying the above mentioned symptoms,a s well as some items that were in draft state. The bugs have been fixed in our Sitefinity 4.1 version, so I would recommend you to upgrade a copy of your project to the latest officially supported version and test if the issue reproduces there (please have in mind that we support up to two versions back upgrade path, so in case you're running Sitefinity 4.0 official release, you should upgrade to Sitefinity 4.0 SP1 and then to Sitefinity 4.1 SP1 and then to the latest officially supported version Sitefinity 4.1 SP3)
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