That's by design. (I assume you're talking about the stat in the lower-left corner.) We felt Avg L/100km was a great stat for US folks to see, even if they aren't used to metric. It's a good way to think about fuel consumption. And we hoped the same would be true for metric folks, that seeing MPG once in a while might be nice.
The top three boxes on the page have your main stats: Average, Last, and Best fuel economy in whichever units you have set. The lower-left box is just an extra.