With other sales and in my case this Supergiant sale:
https://www.gog.com/promo/20190822_promo_launch_supergiant_anniversary
https://store.steampowered.com/bundle/983/Supergiant_Collection/

Its a small percentage and the difference in Pyre is just 1€ (which probably im going to buy here), but why the difference in price? Pyre without discount is 19,99 on Gog and 16,79 on steam. Is the Publisher decision, taxes or Regional pricing?
On The banner saga is the same difference, when buying the complete saga I ended buying it on steam because there it was at 16€ and here 21€. Is not that much but the games are in the end the same.

By the way, i think they should make discounts in bundles for the games you own. Its a nice feature.

What do you think?

Mostly down to regional pricing I believe, and depends on which country you are basing your purchase from (buying a game on Steam/Epic Games Store from a country in Southeast Asia will have different pricing compared to buying in Poland).

However, it isn't always cheaper on the other platforms. For example, due to the Supergiant anniversary promotion, I purchased Pyre, Bastion and Transistor on GOG. I have been waiting on GOG to have this kind of promotion on Supergiant titles, which make them cheaper than buying on Steam (at least for me) even if I could have gotten them earlier (and cheaper) on Steam, simply because they're DRM free on GOG.