re-declaring readonly property in class extensions

Question

I am reading this document to learn objective-C: https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html#//apple_ref/doc/uid/TP40011210-CH6-SW1

I the topic "Use Class Extensions to Hide Private Information" (page 73 of pdf) it says: Class extensions are often used to extend the public interface with additional private methods or properties for use within the implementation of the class itself. It’s common, for example, to define a property as readonly in the interface, but as readwrite in a class extension declared above the implementation, in order that the internal methods of the class can change the property value directly.

What i dont understand in this statement is that since we can change the readonly property from within any private method defined in class extension without that property being re-declared as readwrite in class extension, what does it achieve by re-declaration of that property as readwrite?

Answer 1

1. Actually my assumption that "we can change the readonly property from within any private method defined in class extension" is wrong. Class extension cannot use instance variables synthesized automatically for a given readonly property, because they are private (NOT protected) by default.
2. Even though the document i mention says that instance variable automatically synthesized for a given property has a leading underscore (_Make) in front of it. It actually is not true ( at least not in Xcode 4.6.3 ). It bears the same name has the property itself (unless you synthesize the instance variable itself @synthesize Make = _Make; Please correct me if i am wrong