The deadlifting world record is about 1000 pounds, but that's only for lifting it a few inches off the ground. If you look at Jean Baldwin's CS, you can see the max lifting weight section is meant to indicate how much you can lift above your head, not just off the ground. Thus why he went with 600 lbs as the tier 1 limit instead of 1000. I assume that's the thing you were wondering, as the tiers above 1 are pretty much arbitrary.

As for punching force, i agree it's questionable to use 2x lifting weight for everyone, even those people with significant training. I suppose it's only a rule of thumb though, and the creators of highly trained characters should put the punching force into their CS after all.

Edit: several posts were made while i was typing this, so my comments might be obsolete.