Found this snippet on testing procedures involved in getting your estimated mileage. It's no surprise they differ considerably, as no one on earth drives a vehicle like they test them. At best you can glean relative differences between vehicles. I don't know what the process is south of 49, but I hope it is more realistic.


Testing procedures for vehicle fuel consumption
New vehicles are "run in" for about 6000 km before testing. Vehicles are mounted on a programmable two-wheel laboratory chassis dynamometer and, using two-wheel drive, are run through simulated city and highway driving cycles. This carefully controlled method of testing, including the use of standardized fuels, laboratories and testing equipment, is used instead of on-road driving to ensure that all vehicles are tested under identical conditions.

Fuel consumption ratings are generated based on test cycles and correction factors that take into account the aerodynamic efficiency, weight, rolling resistance, drive mode of different vehicles and average real-world driving conditions in Canada. Other adjustments are made to reflect the average fuel consumption of vehicle configurations, options and sales mixes sold in Canada.

All vehicles, including four-wheel (4×4) and all-wheel drive (AWD), are tested in two-wheel drive (2WD) mode.

Simulated city course
City fuel consumption ratings are based on a 22-minute simulated drive of 12 km, with 16 complete stops. The average speed of the test is 32 km/h.

Simulated highway course
Highway fuel consumption ratings are based on a 12-minute simulated highway course of 16 km with no stops. The top speed during the test is 96.5 km/h, and the average speed of the test is 77 km/h, reflecting an urban-highway driving environment.

Yep, I'm sure this is true.
The estimated mileage for my car will probably never be a reallity, it is closer to 30% more in real life than the factory says.
But OTOH, I'm very impressed by the in-car computer, it is very accurate in its calculation, I think the biggest difference I've ever got was about 7% from the calculated to the real value.

Maybe I am getting old and conservative in my driving style but I have found that our 05 2wd LS gets VERY close to the factory advertised mileage, occasionally better.
Driving race cars for more years than I would care to remember teaches you tricks that will save you fuel as strangely enough many of the things that make you fast on the race track also conserve fuel.
It's all about AVERAGE speed. No stops if at all possible, never fast stops and never jackrabbit starts. Watch WAY ahead and plan which lane to be in to minimize on and off the throttle.
Most of the time this will get you where you want to go sooner anyway. Less wear and tear on everything.
YMMV but that is my experience in 13000 km.
Clark