They don't do full reverse osmosis to the purest extent. There are still quite some minerals left. That's actually better for the end product

I can't speak for coke, but for bottled water, they often add minerals back in.

Honestly, the water is just a guess since the taste is different and the syrup comes directly from Coke. In other comments here, people mention the cans used but I’ve had Coke in glass from different countries taste different.