Well, the standard is..
A1 - 75+
A2 - 70-74
B3 - 65-69
B4 - 60-64
C5 - 55-59
C6 - 50-54
Moderation only occurs when there's too many people getting As or too many people failing as a result of the exam paper being too 'easy' or too ''difficult' respectively. If so, then it may only differ by a couple of points only, nothing drastic.