Thanks, Roan, for responding to my post.  It did occur to me I had hit upon an account renaming but I think something else is happening here.  Let me give an example of an account renaming that seems to be the most common:

User:Penrithpanthers was renamed to User:SATidball -- which you can see at

http://en.wikipedia.org/w/index.php?title=Special:Log&type=renameuser&user=&page=User:Penrithpanthers&year=&month=-1&tagfilter=

04:53, 3 December 2010 Nihonjoe (talk | contribs) renamed User:Penrithpanthers to "SATidball" ‎ (94 edits. Reason: WP:CHU)

If we look up Penrithpanthers and SATidball in the API, using  http://en.wikipedia.org/w/api.php?action=query&list=users&ususers=Penrithpanthers|SATidball&usprop=groups|editcount|gender|registration we get:

<?xml version="1.0"?>
<api>
  <query>
    <users>
      <user name="Penrithpanthers" missing="" />
      <user name="SATidball" editcount="96" registration="2010-09-25T00:30:16Z" gender="male" />
    </users>
  </query>
</api>

The old account name is now "missing" and the new account SATidball picks up the registration date and all the contributions that used to belong to Penrithpanthers. 

What happened with User:Katr67 seems to be a bit different.  We can find a renaming event for User:Katr67 http://en.wikipedia.org/w/index.php?title=Special:Log&type=renameuser&user=&page=User:Katr67&year=&month=-1&tagfilter= shows

03:31, 18 September 2010 Nihonjoe (talk | contribs) renamed User:Katr67 to "Valfontis" ‎ (53,420 edits. Reason: WP:CHU)

However, the User:Katr67 account is not deleted (as was User:Penrithpanthers).  Instead, going to http://en.wikipedia.org/wiki/User:Katr67  redirects  to http://en.wikipedia.org/wiki/User:Valfontis  And we look up both users at http://en.wikipedia.org/w/api.php?action=query&list=users&ususers=Katr67|Valfontis&usprop=groups|editcount|gender|registration we get

<?xml version="1.0"?>
<api>
  <query>
    <users>
      <user name="Katr67" editcount="4" registration="2010-12-10T08:09:19Z" gender="unknown" />
      <user name="Valfontis" editcount="53627" registration="2006-01-06T10:23:49Z" gender="unknown">
        <groups>
          <g>autoreviewer</g>
          <g>reviewer</g>
          <g>rollbacker</g>
        </groups>
      </user>
    </users>
  </query>
</api>

So what's happening with Katr67/Valfontis ?

Thanks,
-Raymond (User:RaymondYee)


On 12/13/10 6:24 PM, Roan Kattouw wrote: 
2010/12/14 Raymond Yee <raymond.yee@gmail.com>:

In computing statistics for many recent accounts, I have these assumptions
to be true for the vast majority of accounts.  I'm puzzled, however, for
accounts in which one or both assumptions are wrong.  An example is
User:Katr67


You probably hit an account rename here. Edits are reattributed for
those, but I guess the editcount isn't updated for that, and of course
the registration timestamp will be different too.

Roan Kattouw (Catrope)