From Sitecore documentation:
__Renderings – a shared field where you specify the common layout for all languages and versions of the item.
__Final Renderings – a versioned field where you specify individual layouts for languages and versions of the item.
The layouts are stored as XML in the fields, and the contents of these fields are merged ("patched") together to create the final presentation that visitors see.
So in your scenario, Shared Layout is merged together with Final Renderings and datasource from Shared Layout is used in the final version of the page presentation.
More can be read https://doc.sitecore.com/developers/90/sitecore-experience-management/en/versioned-layouts.html
And here is the excellent chart which shows how Sitecore resolves final presentation (from the Sitecore documentation link above):