Properly deallocating objects


This may be a dumb question. I’m trying to gain a better understanding of Cocoa memory management, and I’m unable to understand why some objects are released and their pointers set to nil, and another is released but its pointer is not set to nil.

Specifically, why are the calendar date now and the array array released and then set to nil, and lottery entry newEntry only released on pages 69 and 70?