I read your post just now and logged back in to double check, and you're absolutely right that it's got a market cap of 1998 BTC.
Hmm...
According to the API, the only 0.0005-something trade (also the lowest trade) happen during these 3 consecutive trades:
- 2014-01-15 17:55:22 - 1000 @ 0.00080000
- 2014-01-15 18:02:05 - 1 @ 0.00051000
- 2014-01-15 18:52:56 - 1250 @ 0.00080000
You must have looked during the 50 minute period of 6pm (Havelock time) where the last trade would have been @ 0.00051.
At the time the 0.00051 bid was placed and fulfilled, there must have been no bids between 0.0008 and 0.00049999.
This could be disproved if there were any bids in that range placed before the 0.00051 trade happened, but no such bids exist currently.