Flattening (to me) means that there is no real change in maintenance cost savings with increasing thrust reduction. Clearly the numbers don’t show such an effect for the range of thrust reductions considered here. At thrust reductions of 40% the GE/SCNEMA paper indicates the start of flattening of the relation between reduction and cost savings for GE90 engines. Don’t have this information for the CFM56 engines.
The "high" and "low" values are the differences between harsh environment conditions and typical (normal) conditions.