The important thing to remember is that the traction aspect is theoretical. Nowhere does that graph take factor of adhesion into account. It is easy to assume that adhesion is the same as traction. I can tell you that calculating tractive effort on trains, factor of adhesion and tractive effort are not the same. The tractive effort in that case is the amount of torque directly applied to the rails whereas the factor of adhesion is the amount of available grip. Traction on a tire can be thought of in a similar manner since it is still a wheel on a surface.

We can clearly tell that the same amount of weight and same tire area applied to a vehicle on concrete will have a very different level of adhesion than the same vehicle on ice. The overall trend may be the same in regards to weight vs traction but the actual curve will be located at a far different weight number.