Luxury cars have awful gas mileage in most cases.
It was the fifth best in the Large Car sector one year, though. It seems it has really good gas mileage for being a Large car... you need to factor in that "large car" aspect, seeing as they'll never do as well as a Compact and will seldom do as well as a midsize.
A big thing to remember when considering a Buick is that they actually won "most reliable" this year, which is a pretty huge reward to get. =D