If I remember correctly its a consequence of a regulation.
The -10% is probably the first reduction. I think the one with the % needs to show prior reductions. So its probably not the highest reduction the game has.
Both games have already had a similar discount, according to SteamDB.

In Europe, this is what I found:
  • Display the price of the product or service alongside the discounted price, showing the lowest price it has been for the previous 30 days.
Curiously,
  • BG3 doesn't comply to this rule, but it shows the discount correctly on the front store page (and in lists)
  • W40k:RT complies on the product page, but doesn't show the discount correctly on the front page