The tendons tie further up on the bone.
Torque is Fxd or force time the distance the force is applied from the pivot point. That distance is called the moment arm. If the moment arm is increased to 3X its original distance, the same force produces 3X the torque.
This article helps explain more fully.
The question is WHY? I can see why human legs would need to be able to get more completely straight for upright walking. However, why wouldn’t we, evolutionarily-speaking, be better off keeping the original tendon attachment point that our pre-human ancestors had in our arms? Is there some trade-off with precise control of the arms that I’m not aware of, as far as throwing goes?
Chimps and Humans are strong in different ways. Most humans could drag 80lbs across an open field on a hot day. Chimps would give up and overheat. Our short muscle attachments allow for efficient walking in high temperatures. We can roam far in the heat of the day when chimps must rest.Our sweat glands and relative lack of body hair mean we can face the ultimate enemies, the sun and humidity.
Sure, a lot of chimps can out deadlift all but the strongest humans, but can any chimp run a marathon? Or even do the March of Dimes? Sure, a lot of chimps could rip me apart, but I could walk any of them into the ground.
Shorter moment arms trade power for speed. This should be advantageous if your hunting strategy involves javelins.
It doesn’t really have anything to do with strength, chimps just aren’t built to be upright. They can’t fully extend their knees, so any time they’re upright they’re using muscles to support their weight. A human would win a standing contest with a chimp because we don’t have to exert ourselves to stand upright like they do.
A Chuck Norris Style Hidden Chin Fist.
There is in fact what is known as the “throwing hypothesis” of hominid evolution that posits that it was the development of throwing skills that helped lead to bipedalism and the split that produced our ancestors.
Humans throw much better than chimpanzees can.
Source: Previous Straightdope threads on chimpanzees/apes/orangutans/etc.
i can’t help but wonder what sort of advertising was generated by a google search for hairless chimp images 
I thought it was the fact that chimps have rigid wrists that makes them unable to throw with the accuracy that humans do.
As far as throwing speed goes, I’ve heard that chimps can hurl things pretty hard when they want.
I also don’t see how having the older tendon attachment points at the elbow joint would affect tool-making or other fine-motor-skills tasks.
I wonder if not being able to straighten the elbows all the way would reduce the efficiency of bipedal walking, though.
There are a lot of anatomical differences between us and chimps that significantly affect our relative accuracy. Hand, shoulder, forearm, butt, etc. Here’s an in-depth look that someone linked to in a prior chimp-throwing thread: A Prehistory of Throwing.
That still doesn’t address tendon attachment points at the elbow.
I think I figured it out, though. The more leverage you have over the joint, the less precise control you have, and therefore you would throw with less precision/accuracy. Brute force versus very fine control, maybe?