Sierra is a new OS, not a simple update from El Capitan. So, you have to download and install like an app from the App Store by yourself. If you didn't do so, the system will assume you want to stay at El Capitan, no force update, but will give you the 10.11.7 update later (if any).
UpDATES to existing version are shown on the Updates tab. If you want to to go a new version, that's an upGRADE and you find it in Featured or by searching for it. I agree it's not intuitive but been that way forever; OS X Lion was the first version available via App Store and that's how it's always worked. I've literally talked to people who were still on Snow Leopard 10.6.8 and assumed that was the latest available version, because "Check for Updates" said nothing available.