If comprehension and spelling were decoupled but still equally weighted then the aggregate score would've been significantly higher. Let's split the difference and say that 20-30% on the set of [all answers simultaneously spelled correctly AND comprehended] becomes a 25% on the "spells things correctly" axis. If we weight that equally against the "comprehension" axis then you get

  let R = 25   # reading
  let C = ?    # comprehension
  let M = 40   # reported mean

  (R  + C) / 2 == M
  (25 + C) / 2 == 40
  (25 + C)     == 80

  C == 55      # implied comprehension grade from a 40% average

If comprehension was any higher than 55% - which this anecdote certainly suggests - then the 40% aggregate score awarded is an ineffective assessment of the holistic learning picture.

This is the grade school equivalent of tossing out resumes over a single copy editing mistake.