Why do c++ standard not guarantee pointer-inconvertibility between an array of objects and its first element?

According to this

http://ift.tt/2BK7ijO

Two objects a and b are pointer-interconvertible if:

• they are the same object, or
• one is a union object and the other is a non-static data member of that object ([class.union]), or
• one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, the first base class subobject of that object ([class.mem]), or
• there exists an object c such that a and c are pointer-interconvertible, and c and b are pointer-interconvertible.

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast. [ Note: An array object and its first element are not pointer-interconvertible, even though they have the same address. — end note ]

Why do c++ standard not guarantee pointer-interconvertibility between an array of objects and its first element whereas do guarantee class and its 1st member ?

How come this is undefined behavior?

char carr[8];
char& ch0 = carr[0];
auto& carr2 = reinterpret_cat<char (&) [8]>(ch0); // Is this considered undefined behavior?

// Because ch0 (char&) and carr2 (char(&)[8]) are not "pointer-interconvertible".
// So they can not use reinterpret_cast "definedly".
// Array and its first element are not "pointer-interconvertible"
// eventhough they share the same address.