Well, what would you prefer? Should we sit down and calculate the exact amount of strength required to generate X amount of force with varying levels of training? The simplicity I was referring to was the fact that none of us here are physicists. We're not here to create a completely accurate picture of the forces and effects at work in combat situations. We're here to kick ass and take names, and have fun doing it. No one here is going to sit down in the middle of the fight and work out exactly how many joules of energy their punch transfers to the target. At best, they're going to give a general idea of how hard the punch is and just go from there. Setting punching power at 2x lifting strength avoids the absurdity of someone who can lift 2,000 pounds only being able to punch that hard (especially since there are real life fighters who can punch with over a ton of force, but not lift even close to that amount), while still giving characters with less strength yet more training the ability to maintain an even playing field.

The simplicity is not in the reasoning behind the number, it's in the fact that using that number makes the whole thing less complicated to understand and easier to enact.

Though if you meant something else again, please feel free to elaborate.