I am going to say something that I haven't found anywhere but it has been my experience with every single battery replacement I ir somewhat close had.

The new battery, even if it's Samsung original, it isn't as good as new, it is better than the old one, but not anywhere close as first day phone.

Why? No idea. I imagine that although the battery is new it was built circa when the model was in production and somehow that has affected its capacity.

So you get a phone, you pay 60/80 euros, and you get maybe, two hours more of battery when a new phone would give you 6 or 7.