I haven't talked to their engineers on it, but it seems that the 1.28 I have, is a little quicker than the 1.6, and doesn't overfill the bowl as much.
Plumbers will know what I'm talking about here.
Many toilets have an overflow tube where the water from the fill valve goes. This refills the bowl after it's flushed.
Water keeps filling the bowl, until the water in the tank reaches the shutoff level. With most bowls, that means, that not only is the bowl filled back up, it's filled and filled, wasting much water down the drain.
The engineering part I see is,
1) Using just enough for the flush
2) And preventing the over filling of the bowl during refill.