Hello everyone! Looks like we finally got a response from Valve about the rare achievement showcase glitch. (Thanks emil and spockus for pointing this out!)

Source:
https://gtm.steamproxy.vip/discussions/forum/1/1697221160903242085/?ctp=18#c1697221160916755722

"Originally posted by al:
Hi, sorry we haven't replied sooner.

Some data got corrupted in the system that tracks achievement percentages. While we have backups, it's difficult to determine precisely when the stats got reset, or how many achievements were unlocked in between when the backup last ran and when the numbers got reset. The system has been mostly untouched since 2007 and runs on an old database that we don't really use anymore, so it's overdue for an overhaul.

Given all that, we are working on building a new system for tracking the global achievement data. This is almost complete, after which there will be some time while we count up all of the achievement unlocks again (there are billions and billions). Once it's caught up, we'll switch over to reading the numbers from the new system, and the unlock percentages should return to normal.

I'll keep this thread updated on our progress."

(We are still aware of our site having problems. I wish I could give you guys an update but so far we have nothing :( )