The official exchange rate 

  was 6.06 from Jan/03, 2014 until Feb/14
  climbed gradually to 6.14 through Feb/28
  stayed at 6.12--6.14 until Mar/14
  climbed gradually to the current 6.22 through Mar/20
  has been around 6.20--6.22 since then.

Apparently arbitragers have their own "exchange rate" that has been more or less fixed at ~6.10 since Jan/15.
Until Mar/14 that was not far from the official rate.  It is also possibe that bitcoinwisdom was slow to update
the exchange rate in their scripts.