Our de-icing manual touches on the subject but without detailed explanation of why....
"Longer Holdover Times for 75/25 Dilutions
For some brand-specific fluids, protection is increased in some cells when fluid concentration is reduced. The addition of certain quantities of water to some neat(undiluted) fluids can enhance their performance up to a certain point. Without knowing about this particular fluid mix phenomenon, an operator may think that the data presented in the tables are in error."