Not that unusual.
Devs can upload the game, and/or updates to Galaxy,
Devs do not make offline installers, they are made by GOG.

So there's always a delay, between a dev upload to Galaxy, and the release of an offline installer.
We just don't usually see it for new releases, as there will usually be sufficient time before the game goes live, to hide the delay.

Last time, I heard about it, only one GOG employee was creating every single offline installer, on the site, so on a busy day with lots of patches and/or new games, there can be a longer delay, The worst one I've seen was a full day between Galaxy, and offline version.

As the main attraction of GOG for me, is the offline installers.
I'm happy to get them, however impatient, I may be, I'll wait, rather than use a damn store client.