Two weeks ago Apple released updated versions of certain full installers due to expired certificates.
If you want the revised version of El Capitan you can download it from here,
How to upgrade to OS X El Capitan – Apple Support
Go to Section 4, click on Download El Capitan.
The instructions are slightly different from before.
clicking on Download El Capitan now downloads InstallOSX.dmg.
When that has downloaded, double-clicking on the dmg will
mount it an open to the InstallOSX.pkg, which you double-click on.
This will open an Installer window (this does not install El Capitan)
follow the prompts, this will create the Install Mac OS X El Capitan.app in your
Applications folder. you can double-click on that to start the full installation process.
Or you can use the Install Mac OS X El Capitan.app to create a bootable USB installer.
How to create a bootable installer for macOS – Apple Support