Not with an Outback, but with my old caravan it had a MPG calculator. It also didn't match, and even if I simply did the math myself it didn't match. The one on the vehicle is usually an approximation.
The true MPG is revealed over time as you use the vehicle. Plus with the vehicle one, often I would reset it when I did an oil change, to see if it would affect it more during each oil change, and it would. Winter would show lower MPG and Summer higher, but neither would exactly match Fuelly or my own calculations.
Car's computers are just for guidance, even your speedometer will be up to 10% inaccurate! My car's MPG calculator is typically 2 to 3 MPG's higher than "actual" MPG's but you have to brim the tank every time to get accurate data.
I see about a 5 - 6% high reading on my Prius displayed MPG. The MPG you get by calculating the miles divided by gallons (as done by fuelly) will be more accurate over time. Most car displayed MPG's are overly "optimistic", but it gives you a ballpark number.
Over time, yes, but if you want to get the most consistent measurement possible for an individual them always-brimming&venting is more accurate than just filling to the first-click. Pumps can vary significantly as far as the first-click is concerned.