Why tuple inheritance generates zero weakrefoffset while list inheritance generates positive weakrefoffset?

Question

As known, we cannot create weakref to __builtin__ object in python.

import weakref
a = 1
awr = weakref.proxy(a)  # TypeError: cannot create weak reference to 'int' object
a = (1, 2)
awr = weakref.proxy(a)  # TypeError: cannot create weak reference to 'tuple' object

For that the field __weakrefoffset__ refers to availability of weakref, we can see something below:

class TupleInherit(tuple):
    pass


class ListInherit(list):
    pass


print 'TupleInherit.__weakrefoffset__', TupleInherit.__weakrefoffset__
print 'ListInherit.__weakrefoffset__', ListInherit.__weakrefoffset__
print 'tuple.__weakrefoffset__', tuple.__weakrefoffset__
print 'list.__weakrefoffset__', list.__weakrefoffset__
# OUTPUT:
# TupleInherit.__weakrefoffset__ 0
# ListInherit.__weakrefoffset__ 48
# tuple.__weakrefoffset__ 0
# list.__weakrefoffset__ 0

As above represented, list, tuple and tuple inheritor doesn't provide availability of weakref, while list inheritance provides. Result is shown below:

import weakref
a = TupleInherit()
awr = weakref.proxy(a)  # TypeError: cannot create weak reference to 'TupleInherit' object

a = ListInherit()
awr = weakref.proxy(a)  # OK

So, why? May the reason be that tuple is built by __new__? And what if we wanna create a tuple inheritor which support weakref?