<p><strong>I found a clue</strong> on meta SO in a <a href="https://meta.stackexchange.com/questions/125989/strange-unupvote-pattern-across-multiple-users/125993#comment340838_125993">comment posted by an SE employee</a>: </p>

<blockquote>
  <p>We've always removed votes when deleting a user, this is absolutely
  not new behavior, it's happened from day 1. ... The immense
  complication it introduces by not doing that...wow, I don't even want
  to think about that on a weekend.</p>
</blockquote>

<p>In other words, although there are <a href="https://meta.stackexchange.com/questions/125740/dont-throw-away-all-votes-when-a-user-is-deleted">good arguments for not removing votes</a>, we are told there are significant programming difficulties in making that happen.</p>
