title: Incorrect value from dynamodb table description and scan count
body: <p>I'm having a problem with dynamodb. I'm attempting to verify the data contained within,
  but scan seems to be only returning a subset of the data, here is the code I'm using with the python boto bindings</p>
  
  <pre><code>#!/usr/bin/python
  #Check the scanned length of a table against the Table Description
  import boto.dynamodb
  #Connect
  TABLENAME = "MyTableName"
  sdbconn = boto.dynamodb.connect_to_region(
      "eu-west-1",
      aws_access_key_id='-snipped-',
      aws_secret_access_key='-snipped-')
  
  #Initial Scan
  results = sdbconn.layer1.scan(TABLENAME,count=True)
  previouskey = results['LastEvaluatedKey']
  
  #Create Counting Variable
  count = results['Count']
  
  #DynamoDB scan results are limited to 1MB but return a Key value to carry on for the next MB
  #so loop untill it does not return a continuation point
  while previouskey != False:
      results = sdbconn.layer1.scan(TABLENAME,exclusive_start_key=previouskey,count=True)
      print(count)
      count = count + results['Count']
      try:
          #get next key
          previouskey = results['LastEvaluatedKey']
      except:
          #no key returned so thats all folks!
          print(previouskey)
          print("Reached End")
          previouskey = False
  
  #these presumably should match, they dont on the MyTableName Table, not even close
  print(sdbconn.describe_table(TABLENAME)['Table']['ItemCount'])
  print(count)
  </code></pre>
  
  <p><code>print(sdbconn.describe_table)</code> gives me 1748175 and
  <code>print(count)</code> gives me 583021.
  I was the under the impression that these should always match? (I'm aware of the 6 hour update) only 300 rows have been added in the last 24 hours though
  does anyone know if this is an issue with dynamodb? or does my code have a wrong assumption?</p>