Query on im.save in PIL python

I am using im.load() to get the pixels of ORIGINAL_IMAGE. Subsequently, I save it

I see that whatever is saved is different from what I wanted to save -

This is a code snippet and output that will highlight my issue -

Snippet:

im = Image.open(ORIGINAL_IMAGE)
dup_im = im.copy()
pix = dup_im.load() 
dup_im.save(DUP_开发者_如何学CIMAGE)
saved_im = Image.open(DUP_IMAGE)
saved_pix = saved_im.load()

for i in range(10):
        print  pix[0, i], " : ", saved_pix[0, i]

Output of the print -

BEFORE SAVING : AFTER SAVING

$ python test.py 
162  :  162
162  :  162
162  :  163
162  :  163
162  :  163
164  :  162
160  :  161
159  :  160
155  :  153
155  :  155
$ 

Details about my Python version -

$ python
Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin

Any idea why the two fields (pixels before and after saving) aren't the same?

I can recreate this when loading a JPG and saving it as JPG again. The pixel data is equal when saving it as bitmap though. As JPG is a lossy image format, the save causes a re-encoding, thus changing the pixel data.