Here's another interesting paper:
Why Gasoline 90% Distillation Temperature Affects Emissions with Port Fuel Injection and Premixed Charge
Why Gasoline 90% Distillation Temperature Affects Emissions with Port Fuel Injection and Premixed Charge on JSTOR
"However, with PFI( Port Fuel Injection)the fuel is exposed to the air for a short time before it is inducted into the cylinder. This could result in incomplete fuel vaporization, particularly with T90 fuels. On the other hand, with a carburetor and as with TBI the fuel has a long exposure to the air before entering the cylinder. therefore it is conceivable that the mixture is better vaporized and premixed prior to combustion with carbureted and TBI fuel delivery. More complete vaporization could result in better combustion and lower unburnt HC emissions"
There's obviously a lot going on in the intake manifold and port than most people would think. If the fuel required more "energy input" to improve the mixture quality the intake tract could be a good place for that to happen.
