title: MongoDB : query result size greater than collection size
body: <p>I'm analyzing  a MongoDB data source to check its quality.
  I'm wondering if every document contains the attribute <code>time</code>: so I used this two command</p>
  
  
  
  <pre class="lang-javascript prettyprint-override"><code>&gt; db.droppay.find().count();  
  291822   
  &gt; db.droppay.find({time: {$exists : true}}).count()   
  293525
  </code></pre>
  
  <p>How can I have more elements with a given field than the elements contained in whole collection ? What's going wrong ? I'm unable to find the mistake.
  If it's necessary I can post you the expected structure of the document.</p>
  
  <p>Mongo Shell version is 1.8.3. Mongo Db version is 1.8.3.</p>
  
  <p>Thanks in advance</p>
  
  <hr>
  
  <p>This is the expected structure of the document entry:</p>
  
  <pre class="lang-javascript prettyprint-override"><code>{
    "_id" : ObjectId("4e6729cc96babe974c710611"), 
    "action" : "send",
    "event" : "sent",
    "job_id" : "50a1b7ac-7482-4ad6-ba7d-853249d6a123",    
    "result_code" : "0",
    "sender" : "",
    "service" : "webcontents",
    "service_name" : "webcontents",
    "tariff" : "0",
    "time" : "2011-09-07 10:22:35",
    "timestamp" : "1315383755",
    "trace_id" : "372",
    "ts" : "2011-09-07 09:28:42"
  }
  </code></pre>