How, according to, Taplow is Mr. Crocker Harris unlike other masters?

Other masters would certainly given a chap, a form after taking his extra work, but Mr. Crocker Harris is unlike them. When Taplow asked him about his form, he told the boy that he had given him exactly what he deserved: no less and certainly no more. Taplow is afraid that he might have marked him down rather than up for taking extra work.