title: difference between answers using .isna and (df==np.nan).sum().sum()
body: <p>Aftre applying <code>df.fillna(df.mean())</code> to remove Nan values, I am getting different result with <code>df.isna().sum().sum()</code> and <code>(df==np.nan).sum().sum()</code></p>
  
  <p>with <code>(df==np.nan).sum().sum()</code> i am getting 0 
  but with <code>df.isna().sum().sum()</code> i amgetting 33470</p>