Why on hell Python 2.7 (and even 3.3) does not release memory as it should?

gistfile1.txt

Using 2.7.6:

$ python2 ~/mem_prof.py
memory deallocated: 0.0 MB

Using 3.4b1:

$ python3 ~/mem_prof.py
memory deallocated: -1040.09765625 MB

== mem_prof.py ==
import numpy as np
from collections import deque
import gc
import time
import psutil
import os
from six.moves import range

def test():
    n = 2<<25
    a = np.arange(n)
    d = deque()
    for i in range(n):
        d.append(i)
    process = psutil.Process(os.getpid())
    mem1 = process.get_memory_info()[0] / float(2 << 20)
    del d
    mem2 = process.get_memory_info()[0] / float(2 << 20)
    print('memory deallocated: {} MB'.format(mem2-mem1))
    x = np.arange(n)

test()