Thank you @lukestokes, I invited you to https://steemit.chat/group/steeminvestors chat channel.

I don't use the USD price of BTC at the moment of the transfer because I don't have it till now (ok I would need to implement the poloniex or bittrex API to get these prices). But if you move Steem out of Poloniex to your Steem-account, you bought the Steem before the transfer happend with a different price. Same happens if you sell, you move the Steem to an exchange, but you sell the coins later with a different price. As I can not look into the trading histories of the exchanges, I decided to interpolate the prices based on the EOD-prices before and after the transfers.