I reinstalled the GoG client, just to keep my offline installers up to date, and one thing I fail to understand, something that did not change since years, is that the actual offline installer version numbers (the version numbers in the filenames the client is downloading) are completely different from the version numbers displayed in the client / on the GoG website. I genuinely don't understand why that is, if those number would be the same, people could easily make sure their offline installers are up to date all the time.

They usually are the same. e.g. I have Terraria 1.4.0.5 installed, the offline installer is labeled "terraria_enUS_v1_4_0_5_38804". However it's up to developers, and given there's no real standard for versions (sometimes they just use the date), it's not going to be 100% consistent.