When you create a synced article, the article you selected as the source article is marked as the parent article. The synced article you created is a child article.
The child article never has content of its own; it merely points to the content in the parent. So once the shared content article relationship is set up, only the parent's content is ever truly updated; the child just shares that content.
But you won't really notice this in the editor. All the content fully displays in the editor for the parent and any children, and you can make edits in the editor. Those edits are all ultimately saved back to the parent article.
Because of this content mirroring, you won't ever see recent revisions in the child article; they're only visible in the parent. Revisions to both the child and parent will be displayed in the parent's Revision History.