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They are called “part-time” in both referenced articles and various other sources. “Part-time” may mean something related to committee membership to you, but that is not the standard definition. The articles also discuss the “competitive selection” for hiring readers, implying that some of the part-time readers are new. Or more obviously, Stanford had significantly more readers last year than they have ever had before, making it impossible for all of them to have been the same ones from the previous year.
Different colleges have different admissions policies, sometimes very different. One cannot assume that that admissions policies and criteria weighting used at another college is also used at Stanford.
Nobody said GPAs are equivalent in different HSs. Stanford recalculates GPA according to their own system and also considers course selection and how the HS grades students when evaluating GPA.</p>