According to this article, http://www.iclarified.com/62748/how-to-make-a-bootable-macos-high-sierra-usb-ins tall-key,
the stub installer only happens with those not running the current version already.
I am, but I get the stub, too, so maybe it has something to do with it not reporting the correct version.
I got a warning when downloading, but when I started the install, it didn't stop be from installing over 10.13.6, so I would imagine it was 10.13.6, but something in the download process from the App Store recognized that something didn't match in the versions.
You may have to wait until everything is updated on the App Store.