The chemical composition of water changes depending on what you use to heat the water with. Coffee makers have a special heating element that heats the water in just the right way so that chemically it is more ideal to rinse over coffee grounds. It's important to fill the coffee maker with cool water(below 72F) so that the chemical heating process is done correctly and your coffee comes out with the right texture for a solid cup of joe.
Not trying to be dense but how does the chemical composition change? Isn't it H20 the whole time?
A water molecule consists of two hydrogen atoms bonded to an oxygen atom, and its overall structure is bent. This is because the oxygen atom, in addition to forming bonds with the hydrogen atoms, also carries two pairs of unshared electrons. All of the electron pairs shared and unshared repel each other.
The most stable arrangement is the one that puts them farthest apart from each other: a tetrahedron, with the O-H bonds forming two out of the four “legs”. The lone pairs are slightly more repulsive than the bond electrons, so the angle between the O-H bonds is slightly less than the 109° of a perfect tetrahedron, around 104.5°.^2 The heating element in a normal coffee machine keeps this perfect tetrahedron intact while heating the water up. So when the water goes over the coffee grounds with this perfect tetrahedron it chemically can absorb more of the flavor of the coffee grounds.