FYI: similar (exact) results when 64-bit mode:
root@x064:[/data/prj/aixtools/python/python-2.7.12.1/Lib/ctypes/test]../../../python ./runtests.py
A 0x1001f 0
B 0x2001d 0
C 0x3001a 0
D 0x40016 0
E 0x50011 0
F 0x6000b 0
G 0x70004 0
H 0x80018 4
I 0x9000f 4
M 0x1000e 6
N 0x2000c 6
O 0x30009 6
P 0x40005 6
Q 0x50000 6
R 0x6000a 8
S 0x70003 8
.............s....................ssss............................ssssssssssssssssssssssssssss...s.......s.....................sssssssssssssssssssss......s...........s....s.....................FF...s...............................................s.............sssss.ss....s.......ss..sss...................s......s.....................s..................................................s...........ss.sssss................s.s.............s.
======================================================================
FAIL: test_ints (ctypes.test.test_bitfields.C_Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/data/prj/aixtools/python/python-2.7.12.1/Lib/ctypes/test/test_bitfields.py", line 41, in test_ints
    self.assertEqual((name, i, getattr(b, name)), (name, i, func(byref(b), name)))
AssertionError: Tuples differ: ('A', 1, -1) != ('A', 1, 1)

First differing element 2:
-1
1

- ('A', 1, -1)
?          -

+ ('A', 1, 1)

======================================================================
FAIL: test_shorts (ctypes.test.test_bitfields.C_Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/data/prj/aixtools/python/python-2.7.12.1/Lib/ctypes/test/test_bitfields.py", line 48, in test_shorts
    self.assertEqual((name, i, getattr(b, name)), (name, i, func(byref(b), name)))
AssertionError: Tuples differ: ('M', 1, -1) != ('M', 1, 1)

First differing element 2:
-1
1

- ('M', 1, -1)
?          -

+ ('M', 1, 1)

----------------------------------------------------------------------
Ran 440 tests in 0.870s (0 modules skipped)
Unavailable resources: printing, refcount

FAILED (failures=2)