Well…that’s, as you say, a well-conditioned human. The average chimp doesn’t spend several hours a day doing nothing but lifting weights. Yes, it gets more “excercise” than the average human, what with all the climbing, swinging, etc… but it’s nowhere near the level of championship lifters, who spend untold hours not only working out, but eating very high-protein diets, and more often than not use supplements and hormones.
Another factor to be considered is the type of muscle fiber, and how “well developed” it is. There’s two proteins in muscle that work together do the actual contracting, actin and myosin. Myosin has small little “heads” that grab ahold of the actin, and pull it towards the center of the muscle fiber, causing a contraction. If there are more heads on the myosin, or just more myosin in general, you’re going to get a stronger connection, and a larger force per muscle fiber.
This is the reason those two “strongest boys in the world” are so…well…strong. (One is in Germany, the other in the US…both are between 2 and 5, I think.) They both have untold amounts of myosin in their muscle. There’s an enzyme called myostatin that, in a nutshell, eats myosin. One of the boys produces very little myostatin, and the other one isn’t affected nearly as much by the myostatin (I forget which one is which.)