GoG Galaxy 2.0 is not showing the correct percentage for achievements.
Despite unlocking all Witcher 3 achievements, Galaxy 2.0 still claims I unlocked only 99%.
On gog.com, the percentage is even lower with 98%.

The only way I can explain this bug, is if both believe I only have 77/78 achievements:
77/78 = 0.987 => 98.7%
I'm guessing Galaxy 2.0 is rounding the percentage, while gog.com is ignoring the decimal part.

I thought this topic was going to be about a similar issue I've noticed where the rarity of an achievement is off. When steam says that "x% of players have this achievement" I've noticed GOG2 will display a different percentage. Disappointing, given I"ve found no way on Steam to get that informatin for achievements that have fallen off your recent achievement list. I was relying on GOG to give me that information, but it seems to be supplying erroneous data... or Steam is LOL. Seems less likely though.

It seems there are just generally achievement issues based on what I've read.

I've got the same issue with Bioshock Remastered and Lego Batman 3. Although I've all achievements in both games, it falsely shows 98% for Bioshock and 99% for Lego Batman 3.