A small correction here; the 17/24 rating is for the non AWD Traverse LT2. The AWD Traverse LT2 is rated at 16/23. There are some legitimate reasons why your car might be getting 13 MPG, and there are also some possible faults which could cause that. The length of your city trips,.and the ratio of city to highway trips is one possibility. If your city trips are 5 or less miles in length, you will not reach the EPA city mileage figure in city driving. If the number of city trips exceeds the number of highway trips; then the overall average mileage will be weighed down. A second cause could be the quality of the fuel you use. In many parts of the country (especially the midwest), the fuel has substantial quantities of ethanol added to it. If the fuel you regularly use has a high ethanol content; you might find your mileage will increase by using a different brand. Shell gas is known to produce better mileage in many vehicles.
Driving habits and style can also lead to major mileage reductions. Letting the motor warm up before driving is one of the worst gas wasters. It is also recommended against by many car manufacturers, because it leads to increased engine wear; but some people will do it anyway. Driving aggressively, jack rabbit starts and frequent acceleration also are major fuel wasters; along with low tire pressure. Air temperatures below 55 degrees F will further reduce fuel economy.
From reading through the Edmunds Chevy Traverse forum, I see that some owners get or exceed the EPA mileage estimates; while a few others get the kind of mileage you do.
Some of the kinds of mechanical problems which can lead to poor fuel economy are a problem in the engine cooling system which leads to the motor running below its normal temperature; or a defective coolant temperature sensor for the fuel injection system. If there was a problem in the ignition knock (detonation) sensor, which led to the timing being excessively retarded, it would result in a major loss of fuel economy. So would a defective mass airflow sensor or a defective oxygen sensor. These are things which the dealership could check, if they were sufficiently motivated. Sometimes a call to the Chevy regional customer service office would generate more active dealership cooperation in addressing poor mileage. The dealership can temporarily connect a data recording device to the car; to generate an ongoing record of engine control system performance. This can be a valuable aid in locating systems which are not working properly.
There is also a GM TSB (technical service bulletin) issued which could apply to this situation; but I don't have the full text of the bulletin, so I can't be sure if it potentially relates to fuel economy issues. The bulletin is # PIP-4653B, issued Sept 2009 (NHTSA item # 10032434). You might show this information to your dealership.