The review process is completely random. One day they accept your app completely, the other day your "iPad screenshot is a strechted iPhone image" or something. Its just frustrating.
It does seem random - I have submitted apps multiple times, and provided demo credentials for them to use (the app is gated, so you can't do anything without an account and logging in first).
In 80% of the cases they never even logged into the app before giving us the OK (all logins and actions are logged, so it is easy to tell)
I have a pet theory that the reviewers have some quota of rejection that they must meet, so they're encouraged to reject apps for bullshit reasons.
About 20% of the updates to my app get rejected for no reason other than that the reviewer clearly didn't bother reading the attached review instructions. I have a hard time thinking that _all_ of them are this stupid, so nowadays I started thinking they do this on purpose.
In 80% of the cases they never even logged into the app before giving us the OK (all logins and actions are logged, so it is easy to tell)