Hello, i have a question, why does the same game have 3 different discount displayed on the same page?

Well, it should be -10% if you buy only one game from the series. -9% most probably comes from currency conversion and rounding to the closest integer value. -25% applies only when you all three games released today: Jazz Jackrabbit 1, 2 and Epic Pinball.

GOG has never been particularly good at displaying and communicating things to customers...