Water is made up of a single atom (Oxygen) and two atoms (Hydrogen) H20. These atoms coexist as two charged ions in water. A hydrogen atom is a positively charged ion and oxygen is a negatively charged hydroxide ion. Purified water posseses a remarkable solvent capability, resulting in these charged ions to bond with other ions in the same manner. Pure mineral free water naturally wants to bond with minerals, dirt and organic material and therefore becomes an aggressive cleaning agent.
The water purification process removes all mineral and particle impurities enabling it to evaporate without leaving any trace of mineral deposits behind. Water in its purest form (rain water, depending upon the pollution level where you live) has zero Total Dissolved Solids (TDS) in it.
That being said... when water passes through the Deionized Resin filtration system, it contains 0000TDS, therefore making it pure. After the water is purified, it is then forced through a hose to a water fed pole delivery system. It travels up through the pole and to a soft bristled brush. The windows then get cleaned by agitating the dirt on the glass. The water acts like a lubricant to break the bond between the dirt and the glass.
When the glass is clean, a final low pressure rinse is given so that all the contaminants are rinsed away. You should expect to see the windows left wet, this is normal and I assure you they will dry, spot free, leaving only a brilliant shine behind.