title: pandas dataframe sort_values on multiple columns did not work all the time
body: <p>I'm having this 2 pandas dataframes, I tried to sort them the same way, but getting different result</p>
  
  <p>import pandas as pd</p>
  
  <pre><code>df = pd.DataFrame([{'a':'4','b':'23'},{'a':'3', 'b':'10'},{'a':'2','b':'88'},{'a':'2', 'b':'29'},{'a':'2','b':'12'},{'a':'1','b':'284'}, {'a':'1','b':'24'}, {'a':'3','b':'1'}])
  
  print(df.sort_values(['a', 'b']))
  
  df1 = pd.DataFrame([{'c1':'3','c2':'10'},{'c1':'2', 'c2':'30'},{'c1':'1','c2':'20'},{'c1':'2','c2':'15'},{'c1':'2','c2':'100'},{'c1':'1','c2':'11'}])
  
  print(df1.sort_values(['c1', 'c2']))
  </code></pre>
  
  <p>the first one returns, which is expected</p>
  
  <pre><code>   a    b
  6  1   24
  5  1  284
  4  2   12
  3  2   29
  2  2   88
  7  3    1
  1  3   10
  0  4   23
  </code></pre>
  
  <p>the 2nd one returns</p>
  
  <pre><code>   c1   c2
  5  1   11
  2  1   20
  4  2  100
  3  2   15
  1  2   30
  0  3   10
  </code></pre>
  
  <p>I would think it would return</p>
  
  <pre><code>  c1   c2
  5  1   11
  2  1   20
  4  2   15
  3  2   30
  1  2  100
  0  3   10
  </code></pre>
  
  <p>can someone tell me why they behave differently?  Thanks.</p>