The thing about Top Eleven's engine is: some positions are simply more impactful, than others.
AML/R, ST, DMC, DC are the core impactful roles, AMC & ML/R less so, MC & DL/R the least impactful roles.
So training the more impactful positions to higher stars, while keeping less impactful positions at lower stars is an effective strategy to lower your overall % (therefore you're matched against weaker opponents), while the important positions are still strong.
Mutants with high white skills and low (or even highly negative) grey skills follow the same principle of min-maxing.