Which aspect does NOT stay the same across different TLB machines?

The correct choice highlights that weight distribution is a variable that can differ significantly among various Tractor-Loader-Backhoe (TLB) models. Each machine is designed with unique specifications, which may include different sizes, materials, and component layouts. These aspects can drastically affect how weight is distributed across the machine, influencing stability and handling.

In contrast, the center of gravity can be more consistent within a specific category of machines because it usually correlates to the machine's design and configuration. Operator skill level typically varies between individuals but does not relate to the machines themselves. Lastly, performance under load can also be engineered to be similar for machines of the same class, despite other differences in design. Therefore, it is clear that weight distribution is not a fixed characteristic across different TLB machines, making it the correct answer.