What does "average" mean on the Official GMAT diagnostic??

I have recently completed the diagnostic test from the official GMAT Review (13th Ed.). My Scores are as follows:

Quantitative:
Problem Solving - 13 (average)
Data Sufficiency - 13 (average)

Verbal:
Reading Comprehension - 15 (above-average)
Critical Reasoning - 13 (above-average)
Sentence Correction - 13 (above-average)

I have no idea how "average" and "above-average" (according to the attached table) correspond to actual test scores. I write the exam in about 3.5 weeks
...

Attachments File comment: GMAT Diagnostic Score Chart

2FB85D77-F6EF-4C3A-A3EB-609B21364DF0.jpeg [ 182.67 KiB | Viewed 51 times ]

---