The fuel in MPFI is injected onto the back of the valve while it is closed so the fuel is the first thing in as the valve opens. Think of it like putting cream in your coffee, if you add the cream to your cup first(small amount) then add the coffee(larger amount) then just the act of pouring the coffee in will mix it all up and you don't need to stir it at all, on the other hand you pour the coffee first then squirt in the creamer you are gonna have to stir it because it won't mix on it's own.

Carbs on the other hand spew the mix in the chamber at the same time, it also has more time in the runner to evaporate a little more or for the fuel to seperate out if the manifold or port is poorly designed.

EFI also comes out at a much higher pressure and this also helps to atomize so 6 of 1 or 1/2 dozen the other way.

Direct injection is more about MPG and emmisions than power or even atomization, it allows the whole combustion chamber to filled with a homogenious mix under WOT or more importantly, part of the chamber can be filled with a real good mix near the spark plug while the rest is just air, this minimizes heat rejection into the metal because the air acts like insulation, you can intentionally have lean and rich areas at the same time in the chamber to improve MPG and emmisions, this is not really utilized under WOT because a carb or port injection both can do a fine job of filling the entire chamber with a good mix.

In my personal opinion if MPG were the primary concern you would want all the fuel vapoorized and evenly mixed so you would want the high pressure fuel atomization of MPFI and a longer time in the manifold that you get from injectiing it at the beggining of the runner instead of at the valve, my TBI motors I have owned always got the best MPG, chevy, dodge both got better MPG with throttle body injection than MPFI magnum and vortech style engines at the cost of power.