Memoria : 50 PT but only 20€ ?

It is a well know bug. Some Daedalic package/bundle is on sale on Steam, with a price below 20€. So this is the cheapest possibility to buy Memoria => the base price of that package is reported by Steam's API to Steamgifts. Will be fixed, once the deal is over.

The number of points being based on Steam's price

Correct. And if you look at Steam's price, you'll notice that Steam bugs out during sales (in any instance where a package [sharing a store page with a base game] goes on sale cheaper than the base game's price).

This is purely on Steam's end, and you can notice the same thing within Steam searches, checking your wishlist, etc- the incorrect price is consistently displayed across Steam during a sale. SG syncs often enough that it'll update near-immediately to the actual value of the game when the sale ends.

Thank you for your answers, I think (new ?) users have now enough explanations on this :) I close this thread.