Hill mentally calculated Russell's learning speed... Given how the library inside that ring was used by this kid, he was aware of it.
Although he left a margin for himself, this was indeed Russell's limit.
However, Hill could still see some lack of confidence in this child.
Among these books, at least half were very familiar to Russell, something he could definitely recite.
But he clearly planned to go through them once more, just in case he missed anything.
Hill is someone who has great self-confidence, so he didn't quite understand why Russell seemed so unaware of himself... But he never interferes with others' self-awareness.
That is not something outsiders can involve themselves in, even one's parents can't; he is just a teacher after all, his role is to point out the path for the other to take.
Hill is never someone willing to shoulder responsibilities beyond his tolerance; he likes clear divisions of duty.
