The scanguage II gets its information from the ecu via the can-bus just like the instrument cluster on the dash, and as such, is no more accurate. The only real way to properly gauge average fuel economy is by comparing the litres or gallons put in vs. the km or miles since the last fill up.
I will say that while the Kia we own doesn't get the claimed fuel economy that was on its window sticker, at least the speedometer doesn't read 8% higher like the '08 Mini Cooper S we traded in. The scanguage is only as accurate as the ecu, which is never more accurate than the sum of its sensors and calibration.
I'm one of those people affected by Kia's bogus mileage claims the most with the 6mpg difference. I traded in a Mini Cooper S for a optioned out Kia Soul Exclaim Premium. Both cars had the same ratings according to the window stickers, 28 city, 34 highway, EPA ratings have gotten significantly better (with some obvious exceptions) in recent years. I typically get about 2mpg less than sticker due to my driving style, the crappy 15+% ethanol laden gas, and the hilly terrain around where we live, but on longer drives the highway mileage always lines up if I compare the gps-read-miles and the gallons out of the gas tank. True to this finding, our Soul has been getting 6mpg less than it's rated and other owners on the Kia Soul forums say the exact same thing.
Really it's common sense and I knew the window sticker wouldn't be completely correct when we bought the car. Any time you see claims that a companies top of the line trim with much bigger, wider wheels, a larger engine, and an automatic gets the same mileage as their entry level trim with the little engine, skinny tires, and a stick shift, you know someone's not doing the math. I am a little shocked to see the EPA actually picked up on this and delivered a solution. Kudos to the guys in charge for once. I'm also pretty happy with the solution. I know some Mini-zealouts will rage on me for saying this, but our 2012 Kia is 100x nicer to drive around the city and on longer road trips than our Mnii was and it's even better now that they both are on equal (effective) gas mileage terms.