MPG will always be different. Different day to day, and different between 2 identical vehicles.
Because there are too many variables involved..
Rolling resistance = Tires Size / tread depth / brand/
Current Wind speed
A/C ON/OFF?
Windows = Open /Closed?
Auto / manual?
Cruise Control?
Other accessories, how much is running?
How much stereo equip do you carry around?
Road surface.
Road elevations - Hilly? Flat? Somewhere in-between?
How clean is your car's exterior? Dirt film or fresh wax?
How much extra stuff are you carrying around?
How much do you weigh?
Sunroof open?
Rear spoiler?
When do you shift?
and on and on...