Mechanical advantage and impact. Weight is not just a constant force in our joints, and it's not applied evenly. As an example, imagine holding forty pounds at arms length vs wearing it in a backpack or letting it dangle to the floor. The forces on the shoulders from all three are radically different.

Additionally, when you walk or run you don't place only and exactly the force of the weight of your body down. Each foot is loaded with an impact and the forces are distributed up the leg. What your knee experiences is a dynamic and spiky load.

. . . "A weight reduction of 9.8 N (1 kg) was associated with reductions of 40.6 N and 38.7 N in compressive and resultant forces, respectively." . . .

. . . "Our results indicate that each pound of weight lost will result in a 4-fold reduction in the load exerted on the knee per step during daily activities. Accumulated over thousands of steps per day, a reduction of this magnitude would appear to be clinically meaningful. " . . .

https://pubmed.ncbi.nlm.nih.gov/15986358/

I assumed it was a typo - 10%. Though the math still doesn't work out for the 40lb reduction of weight on joints.

It's one of the "true but over simplified".

Remember that pounds is a unit of force - not mass.

There's the set of "tech neck" images (example https://www.vital-balance.com/en/tech-neck/ though many more can be found) where it shows what the force on the neck is from the head. At 0°, it's 10-12 lbs of force down. If you've got your head tilted at 45° looking at a phone, the infographic says that its 49 lbs of force on the neck.