title: Wrong decimal calculations with pandas
body: <p>I have a data frame (df) in pandas with four columns and I want a new column to represent the mean of this four columns: df['mean']= df.mean(1)</p>
  
  <pre><code>  1    2    3    4   mean 
  NaN  NaN  NaN  NaN      NaN  
  5.9  5.4  2.4  3.2    4.225  
  0.6  0.7  0.7  0.7    0.675  
  2.5  1.6  1.5  1.2    1.700  
  0.4  0.4  0.4  0.4    0.400 
  </code></pre>
  
  <p>So far so good. But when I save the results to a csv file this is what I found:</p>
  
  <pre><code>5.9,5.4,2.4,3.2,4.2250000000000005
  0.6,0.7,0.7,0.7,0.6749999999999999
  2.5,1.6,1.5,1.2,1.7
  0.4,0.4,0.4,0.4,0.4
  </code></pre>
  
  <p>I guess I can force the format in the mean column, but any idea why this is happenning?</p>
  
  <p>I am using winpython with python 3.3.2 and pandas 0.11.0</p>