I've installed Batman: Arkham Origins and the Season Pass from the GOG offline installers. But launching the game from the GOG Galaxy (which is the default if you press "Launch" at the end of the installer), starts downloading an extra Update of ~3.3 GBs.

Why has this extra update not been included in an updated offline installer?

I had this also with many other games. Seems like Galaxy does not really check or care about DLC you installed with offline installers. It installs it again no matter what.

Interesting. I did not consider this case. But it makes sense.

Hopefully, Galaxy will fix this issue in the future to prevent confusion.