There exists what is known as an ideal gasoline-to-air mixture.  Any
deviation richer or leaner has a negative effect on mileage/efficiency.
Electronic fuel inj. continuously monitors this ratio and alters the
mixture, many times per second, to keep it at this ideal.  Carbs just cannot
achieve this accuracy, even with computor-monitoring and altering.  Also,
carbs put the mixture into the general proximity of all the cylinders,
meaning that with all 4 cylinders receiving fuel from the same source, some
cylinders will get more, some less, depending on its distance from the carb.
With elec. fuel. inj.--esp. multi-port inj.--each cyl is the same distance
from its injector and can receive the same, theoretically exact,  input of
fuel on each pulse of the injector:hence a second important feature for
economy.
   The throtle-body injection, esp. before the "Vortec" type that GM has,
has the similar precise control of mixture, but lacks the
equi-distant-to-cylinders characteristic of multi-port.  Its efficiency
probably lies somewhere between carbs and multi-port.
   HTH, s