To reproduce the error:

$ wget http://dbnsfp.houstonbioinformatics.org/dbNSFPzip/dbNSFP2.0.zip

$ python
Python 2.7.4 (default, Sep 26 2013, 03:20:26) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from zipfile import ZipFile
>>> 
>>> with ZipFile("dbNSFP2.0.zip", "r") as zf:
...   with zf.open("dbNSFP2.0_variant.chr20", "U") as f:
...     count = 0
...     for line in f:
...       count += 1
... 

>>> print count
964352

$ unzip -p dbNSFP2.0.zip dbNSFP2.0_variant.chr20 | wc -l
2161277

may it be related to issue 6759 ?

Could you please test with in-development version Python 2.7.6+? Unlikely it is related to issue6759, but perhaps it is related to issue20048.

With 2.7.6+ works fine.

Python 2.7.6+ (2.7:c28e07377b03, Jan 22 2014, 12:56:14) 
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from zipfile import ZipFile
[54979 refs]
>>> 
[54979 refs]
>>> with ZipFile("dbNSFP2.0.zip", "r") as zf:
...     with zf.open("dbNSFP2.0_variant.chr20", "U") as f:
...             count = 0
...             for line in f:
...                     count += 1
... 
[55930 refs]
>>> print count
2161277
[55930 refs]

I tested it with Python 3.3 and works fine too.

Thank you for your report Christian. This bug already fixed in issue20048. Wait for Python 2.7.7 bugfix release.