title: GROUP BY of unique users' contributions returns higher totals than expected
body: <p>Using the <a href="https://data.stackexchange.com/" rel="nofollow noreferrer" title="Data.SE">Stack Exchange Data Explorer</a> I'm trying to figure out who the top ten answerers on Stack Overflow are. This is my query:</p>
  <pre><code>SELECT TOP 10 COUNT (PostTypeId) postcount, OwnerUserId
  FROM Posts
  WHERE PostTypeId = '2' AND OwnerUserId IS NOT NULL
  GROUP BY OwnerUserId
  ORDER BY postcount DESC
  </code></pre>
  <p>And, these are my results:</p>
  <pre><code>postcount OwnerUserId 
  --------- ----------- 
  23628     22656       
  17505     29407       
  12978     157882      
  10863     34397       
  10598     23354       
  10560     17034       
  9341      20862       
  8600      115145      
  7394      57695       
  7073      1583  
  </code></pre>
  <p>The problem is: User 22656 is <a href="https://stackoverflow.com/users/22656/jon-skeet">Jon Skeet</a>, and his <a href="https://stackoverflow.com/users/22656/jon-skeet?tab=answers">profile says he has 23,702 answers</a>.</p>
  <p>I checked the others: <a href="https://stackoverflow.com/users/29407/darin-dimitrov">Darin Dimitrov</a> is User 29407. His profile indicates he has <a href="https://stackoverflow.com/users/29407/darin-dimitrov?tab=answers">17,525 answers</a>, not merely 17,505. The others are all similarly skewed.</p>
  <p>Why am I getting wrong totals in <code>postcount</code>? How can I fix this?</p>