I haven't looked into the DB yet, but my guess is that a large part of these 'new hosts' comes from the 'e-science GRID', where every node of every cluster they submit an Einstein@home job on is potentially recorded as a new host at the server side (currently this 'participant' has ~250.000 hosts registered).
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Amazing Numbers of New Hosts
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I know that they now have their own management of hostids, but apparently this didn't work always as it should (remember the monster-host?).
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