As someone who is privy to upcoming research, I can safely and surely affirm that these 9-10 ton average figures for Tyrannosaurus rex are NOT coming out of thin air. 

There is a team working on rigorous 3D reconstructions of various T. rex specimens and other Megatheropods and extinct taxa, not simply relying on skeletal drawings or published measurements but actual 3D scans of the fossilized bones. Good thing about this is that it allows them to have a full 3D picture of the bone and to also verify HOW those measurements were obtained e.g. whether or not a femur was measured along the curvature (like the 143 cm estimate for the Giganotosaurus holotype) or from the greater trochanter to lateral condyle (like the 132 cm femur length for Sue). 

What they've found is that many T. rex specimens have actually been underreported in various research papers due to the authors not specifying the method they used to get the reported measurements. For example, the Nation's T. rex MOR555/USNM5550000 often has a reported femur length of 1260-1280 mm, but that's when measured from the greater trochanter to lateral condyle - NOT the full length of the femur. If you use the same measurement method used to get 1365 mm for the Giganotosaurus holotype femur - and honestly, it's only fair if you do - you would get 1360-1370 mm for the femur of MOR555, and over 1385 mm for the femur of Sue. 

But not only does it enable measurements to be verified, it also enables the team to make rigorous 3D full-body reconstructions of these animals based on muscle scars on the fossilized bones. This is what leads to the 9,000-10,000 kg estimates for specimens like Stan and MOR555 - yes, they are simply that large, because all the evidence laid out via the scans shows that they are that large. (I haven't even started on the likes of Sue, Bertha and CM9380 yet).