I have two games less listed in my GOG Galaxy 2.0 library count than in my GOG.Com website library count, and I have no idea why... There is no obvious reason why this should be the case because the GOG Galaxy 2.0 client should always have at least as many games listed as on the GOG.Com website (I don't have any non-GOG games listed in the client but even if I had, the client would have more games, not less as is the case here). I don't have any preorders of unreleased games that could be messing up the count in either direction so there really should be no discrepancy. I could of course manually compare the two lists to find out which ones they are but with hundreds of games that becomes a laborious task...

At first I thought that maybe this discrepancy could be due to different handling of bundled freebies between the website and the client, such as for instance "Neverwinter Nights Diamond" which was included in the purchase of "Neverwinter Nights: Enhanced Edition" and which lacks a dedicated store page, but a simple check proved that both libraries include "Neverwinter Nights Diamond" as a standalone game in the library count.

So, to sum it up: no non-GOG games added, no freebie game discrepancies, no preordered games, nothing apparent at all that could be messing with the game count - and, yet, the game count is not the same between the website and the client. These two missing games in GOG Galaxy 2.0 are a real mystery to me...

I think I've figured it out... First I counted the games manually and established that the game counts accurately represent the number of games listed at both GOG.Com and GOG Galaxy 2.0, i.e. that the discrepancy of 2 games was real and not just a summing error. Given this, I then went through the list manually and these are the things that I found in each library that were not present in the other - a total of 4 items at GOG.Com and 2 items at GOG Galaxy 2.0, which explains why there are 2 more items in my GOG.Com library than in my GOG Galaxy 2.0 library:

GOG.Com:

Leisure Suit Larry 1 (VGA) - In the Land of the Lounge Lizards [PART OF THE NON-VGA VERSION?]
Leisure Suit Larry 6 (VGA) - Shape Up Or Slip Up [PART OF THE NON-VGA VERSION?]
Ultima VII - The Black Gate + The Forge of Virtue [PART OF ULTIMA VII - SERPENT ISLE + SILVER SEED?]
Ultima 4: Quest of the Avatar [DUPLICATE OF ULTIMA IV?]

GOG Galaxy 2.0:

Cognition: Game of the Year Edition [GOTY EDITION COMPRISES PARTS 1-4 WHICH ARE ALL LISTED SEPARATELY]
Shadow Warrior (2013) [OBVIOUS DUPLICATE ENTRY]

So, there is one apparent duplicate (Shadow Warrior), another possible/probable duplicate (Ultima 4), two VGA versions of games that might be present by way of their non-VGA versions even though they are not listed separately (LSL 1 and 6), one part of a game package that could be listed under the other half's name (Ultima VII), and on Game of the Year Edition that's erroneously listed separately from its component parts 1-4 (Cognition).

In other words, it seems at first glance that the discrepancies are all due to inconsistencies between GOG.Com and GOG Galaxy 2.0 in the way they are sorting these titles.