contamination from condensing pipes, filtration after condensed, air quality, water memory...
Distilled water also reacts with atmospheric carbon dioxide according to
H2O( ) + CO2(g) H2CO3(aq)
H2CO3(aq) + H2O( ) H3O+(aq) + HCO3-(aq)
HCO3-(aq) + H2O( ) H3O+(aq) + CO32-(aq)
which brings the pH of distilled water in an open-air container down to about 5.8.
Pure distilled water is NOT easily obtained because carbon dioxide in the air around us mixes, or dissolves, in the water, making it somewhat acidic. The pH of distilled water is between 5.6 and 7.