I suggest you do three things;
1, insure your tires are propperly inflated. I keep mine inflated a little high to get better gas milage.
2, Figure your gas mileage manually and compare it to what the MPG display says, the two numbers should be close to the same.
3, Take this information to your dealer service manager and see about having the problem repaired under warranty. If the two numbers are not close, or your milage really is 15 to 16 MPG, the vehicle has a problem and I think it should be repaired under the warranty. Your fuel milage should be close to the EPA estimate. My Sorento GDI milage is close to what the sticker said.
What is the terrain like where you live/drive? If there are lots of hills and curves, it will significantly lower your mpg. The 4 cyl. engines in the Sorentos work very hard when pulling hills. There are a lot of hills in my area, and the V6 models get the same or better fuel economy because they don't work as hard. Also, how much weight is carried in the vehicle makes a huge difference. If you are constantly hauling around 1k lbs of weight, it will significantly lower mpgs, especially with the GDI.
Also ethanol content in the fuel makes a difference. In some states, all gas is 10% ethanol(E10), which will slightly lower mpgs.
The three things that lower mpgs that are easily changed, are speed, tire pressure, and weight in the vehicle.
what one person gets, does not mean that another will get it unless you live in the same neighbourhood and drive under that same identical conditions. So saying you have a problem may not necessary apply. There is so many factors effecting fuel mileage.
OP: you get 15 mpg driving without traffic but are you driving a lot through residential areas with numerous stop signs ? in the city, do you encounter a lot of traffic lights?
I live in a city where my driving conditions include a lot of the above in addition to being in stop-and-go traffic and I typically get about 11-15 mpg with my 2.4L GDI. On the highway, I can get around 25-27 mpg if I respect the speed limit.
I think the poor mpg I have is due to the weight of the vehicle, it takes a lot of energy to move this heavy vehicle from a stop.
When our 2012 GDI AWD is used as a golf cart, crawling around our village at 8 mph. Mileage, 17. It used to be 15, but it's breaking in.
I've improved our mileage by modifying my driving. The Sorento coasts a mile when throttled off. So, I get up to speed, then coast all the way to the next stop. My wife hasn't learned this trick. "Go" means throttle on until "Stop", then "Screech" heavy braking at the last moment. This puts her right at 15 mpg.
On mostly flat, well slight ups and downs traveling between San Jose and San Francisco, mileage jumped to 27 mpg.
The Sorento is nearly as big as the Honda Pilot or Toy Highlander, which get less mpg.
There's a lot of E10 out there. My 50 mpg motorcycle will drop to 34 mpg when I put E10 in the tank. This is why mileage varies so much between High ethanol states and "no" ethanol states.