It is a very interesting topic.
You will know the reason, after understanding the knowledge about kinetic energy, chemical energy, thermodynamics, electrical energy.
Basically there is a nonzero threshold operation in gasoline auto, however we can have zero output for Electric Vehicle during static motion in city to save energy.
Also DC motor efficiency in EV is over 80 % and recover energy for the farther distance via chargeable device during running in city, while gasoline auto provides 40 % kinetic energy ultimately in highway.
In city, gasoline auto gets lower energy efficient due to slower speed and have nonzero threshold operation, energy waste due to traffic or control signals. Hence auto overall efficiency will be lesser than 40 %. MPG is smaller than in highway accordingly.