I see no reason why Mitsu would claim their car is heavier than it really is. Assuming Mitsu's scale is correct, what you're seeing is fluid level differences or out of adjustment scales. The difference between a full tank and a 1/4 tank is 60-65 lbs.

Edited to remove HTML crap.

 

If these weights are accurate then the scale I used to weigh my car was off. It weighed in at discouraging 3450 lbs with me in it and a half tank. The scale was probably inaccurate though. Who knows when it was calibrated, it was at a track.