title: MongoDB collection toArray() length is 20 less than collection.count()
body: <p>I am using mongoDB version 3.6.3 on a ubuntu operating system.
  I have created a collection with 100 records</p>
  
  <p>To manipulate the data on the mongo shell I assign cursor like below</p>
  
  <p><code>cur = db.dummyData.find({}, {_id: 0})</code></p>
  
  <p>now the <code>cur.count()</code> is 100 but <code>cur.toArray().length</code> is 80.</p>
  
  <p>I not sure why this is happening. I have tried with bunch of different collections toArray() length is always 20 less than the actual count.</p>
  
  <p>Would appreciate any help to understand this behavior.</p>