2009 Toyota Sienna Mileage Less Than Advertised.?

I recently bought a new 2009 Toyota Sienna two months back. When driving it out of the dealer lot it had 130 miles. It has a instrument panel which gives the average mpg. Out of the dealer lot it showed 14 mpg. Now it has increased to 15.2 mpg at 790 miles and has stayed there for a long time. It is not giving the average of 21 mpg as advertised. I asked the service center about it and they said it will increase after 500 miles. I am having my doubts now. Does anyone have similar experience?