Hercules
Count Of Durfisto
I would term a "base level" as a weight that is well within your limits, something you would use to work out with in the gym.
Example someone can bench say 200lbs for say 10 reps then it gets hard, 200lbs is there base level.
However their one rep max is lets say 270lbs, this is them pushing it to the limit.
70lbs doesn't sound much but next time your in the gym add say just 20lbs to the wieght you usually bench and feel the difference.
And with Superhumans were talking in tons not lbs, then you add in other factors like high stress levels, anger etc etc.
There are plenty of stories of say 100lb women suddenly finding the strength to pull a door off its hinges to get to there kid in a fire.
or someone being able to move rubble that weighed 100's of lbs to rescue someone trapped underneath.
Adreniline is a big factor in strength levels for normal humans, no reason it can't work the same for Superhumans too.
This would be a "real world" answer to why strength levels can vary so much and what base line strength could be.
You add into that, that writers are inconsistant and that this is fantasy so a character can do whatever the writers imagination (or lack of it) wants them to do.
All these factors are why its so tough for the handbook writers to get anything acurate!
I think the handbooks are a good guide line and if a character does something consistantly in his/her book you can build from there.
Class 100 characters can toss 100 tons around like I would use 100lbs for a warm up set on my bench.
Its nowhere near their max, its not even remotely near, but 100 tons is as good a place as any to start.
Hope that made some sense!