Many GOG game updates have no official update flag.
I started to record it since Nov 2017. Only games I have are listed here. 
Report for Dec 2017:
Total updates: 105
Games updates: 80 (Some games have multiple updates, daily basis)
Flags I got: 25 (One of them delayed 7 days)
So, only 23~32% updates have an official flag. 68~77% are missing.
 Dec 2017 record: https://pastebin.com/4u8hCu98
 Nov 2017 record: https://www.gog.com/forum/general/the_what_did_just_update_thread/post18164
Since I recently received quite a lot of flags your numbers seemes to be a bit high for me. So I took the liberty to check those updates against the reported updates. Several of those "Updates" were only installer updates and the game was not changed at all. I would not want to have an update flag for those. A few of them are Galaxy only updates. I took your December List and got those REAL updates with missing flags as end result:
20171229 titan_quest_anniversary_edition (First update was flagged fixed update was not flagged)
20171228 divinity_original_sin_enhanced_edition (Linux Update)
20171219 oxenfree (Galaxy Update)
20171218 beat_cop (Linux/Mac Update)
20171213 grim_fandango_remastered (Galaxy Only Update)
Makes 28 missing update flags. Still far from perfect but completely different numbers. It's fair to pint out what GOG has done wrong so they could do it better but why should I complain about an update flag where nothing has changed or when the changes only matter for Galaxy (which has its own system of "flags")?