You will ALWAYS get a lower indication for short distance/time because again, it's a running average. At the very least, it's best to calculate actual mileage manually as you refill the tank to full with the miles since the last fill to full. The onboard system is good for a general indication, especially over time. For example, my Ascent is currently showing 19.3 as the running average. That's lower than it used to be when we lived farther out of town, but trips are not shorter. "Most folks" are going to have a general overall average of about 19-20 mpg on the Ascent. Heavy urban drivers will get lower. Folks who spend a lot more time driving "uninterrupted" and at steady speeds either on regular local roads or on the Interstate will get more. For highway travel, the range (in general) seems to be 24-27 mpg, depending on the driver, terraine /weather and the speed they choose to drive. I typically get not quite 27 on the highway when traveling...not heavily loaded with one or two people in the vehicle and with speeds of 65-70 mph. I rarely drive at speeds above 70 mph unless I'm actively passing someone and need to do so for time/safety reasons.
Remember, the Ascent is a nearly two and a half ton vehicle in the shape of a brick. It's not going to be as efficient as a small sedan that sits low to the ground and weighs almost a half ton less.