It is because the dilution alters the fluid viscocity....and the 75% can now absorb more contaminant before failure. Also, the fluid is diluted with hot water and this, in turn, imparts heat energy to the airframe. The diluted fluid has a better min HOTcompared to the undiluted fluid but the max holdover time generally is shorter.
There you go, whiteknight, wont have to mention this, now, in your 380 RTGS