?
You can always run the full installer, if you want, or if you need to do that.
I suspect that you are just seeing an update for El Capitan. Current version is 10.11.1
What version of El Capitan are you running NOW? You can check under the Apple menu, About This Mac.
On that information window, you can click on the Software Update... button.
That choice will take you to the App Store/Updates tab, and show any updates that might be offered.
You can go ahead and accept/Install whatever updates that appear.