In the '08 manual, go to page 73 - Driver Controls. It shows you how to reset the "oil life" percentage. It starts by resetting it to 100%, but if you keep hitting "Reset" it will drop it down 10% each time you push the button to get to the percentage you want (rounded to the nearest 10 of course).

You actually need to do this manually at each oil change - basically it's just a glorified trip odometer.

I'd bet that someone who test drove it messed around with resetting the percentage and maybe left it on 80% before you picked it up.