title: different decimal points after doing pandas series calculation
body: <p>I'm trying to do a calculation within pandas series. There is no error in the code.</p>
  <pre><code># create pandas series using dictionary
  student1 = pd.Series({'science':100, 'english':80, 'math':90})
  student2 = pd.Series({'math':80, 'science':90, 'english':80})
  
  # calculate by score
  addition = student1 + student2               
  subtraction = student1 - student2            
  multiplication = student1 * student2        
  division = student1 / student2               
  
  # series to dataframe
  result = pd.DataFrame([addition, subtraction, multiplication, division], 
                        index=['addition', 'subtraction', 'multiplication', 'division'])
  print(result)
  
  </code></pre>
  <p>And the outcome is as below.
  I wonder why do all columns' decimal points are having different outcome.</p>
  <pre><code>                english      math      science
  addition          160.0   170.000   190.000000
  subtraction         0.0    10.000    10.000000
  multiplication   6400.0  7200.000  9000.000000
  division            1.0     1.125     1.111111
  
  </code></pre>