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 and anything above 550 is fine. Am I on track or am I screwed?