Real World lifting capacity, using the lifting capacities for Strength Scores in 3E and 4E, put the strongest real world man at 23, and the strongest real world woman at 21.
Olympic weight lifters are very specialized, so I would just leave those who are in that small fraction of a percent of humans outside the normal ability rules. (Maybe they have a feat that increases their strength by X for the purposes of lifting stuff.)
However, if you want them in the rules, I don't see how an adventurer would through ordinary course of adventuring (i.e. levels) advance from Str 18 to 23. That sort of thing takes years of dedicated training and would be better modeled outside normal level progression.
Alternatively, just adjust carrying capacity to make Str 18 a more accurate cap. My point was that 3-18 should be defined as the human range and all other numbers should derive from that.