Right but the later questions are harder than the earlier ones.

From page 4:

  Problem difficulty increased across the experiment from one-step1 (problems 1–4) to two-step2 (problems 5–8) to three-step3 (problems 9–12).

From page 6:

  Problems increased in difficulty from one-step (problems 4-6) to two-step (problems 7-10) to three-step (problems 11-14) problems.

The fact that the AI group did worse on the harder problems than the non-AI group did on the easier ones doesn't tell us anything.