I asked a reviewer on the phone once. He said this:

There are 3 levels of review (Technical, policies and X). Random reviewers test different aspects. There is also 'Escalation', which is when the reviewer is not sure and the internal policies are unclear.

Furthermore, he says that the policies in the review team change every week sometimes. Examples of policies:

- No women with things in their mouths are allowed

- If a butt is shown, a reasonable amount of torso has to be shown

- You cannot make fun of religion, you can only talk about religion in a neutral manner

So the reviewers have a long and complicated list of rules they are to follow, and this rule list is dynamically updated when an app comes in with new type of offensive stuff.