A very strong test score alone rarely qualifies for the top merit band, and a strong transcript alone rarely qualifies either. The merit committee is looking for consistency, someone who's built a strong academic track over years and confirmed it under exam pressure on the day.
If your transcript is average but you clear the section cutoffs comfortably, you're still in the merit conversation. If your transcript is strong but the test came in weak, retake the diagnostic and see whether a second sitting is possible, some applicants recover meaningful merit by re-testing.