[akdavis]

Well, for testing they didn't fully load it up with tanks and everything, now did they?

They most certainly did. Read the graphs. It's right there in front of you.

NA test: gross weight 9611 lbs. (but here 442mph is the calculated speed for 8,000 lbs.)
AAF test: gross weight 9760 lbs.

Those are combat take-off weights. Anyways, the table you brought forward is clearly nonsense. The dimensions are wrong (or at least oddly presented), the HP is wrong, the weights are wrong (probably P-51B), and now that I look at it again, the climb data is wrong (10 min. to 20000 ft??). The speed data conforms to two actual flight tests of P-51Ds. This is not surprising, as you could probably pick 5 random "stat" tables like this from various books and they would all have different numbers.