Probably because it’s a very rough calculation. He talks about dividing 100-300 trillion by 20 million. The extra million wouldn’t make much difference in his Jan 2009 prediction guesstimate of $10 million per coin. 

 It’s interesting as obviously a million were never moved 

 If you had been planning to mine a million coins and never move them, would you hint about that in such a way? I’d say it was just a simpler calculation.