I believe you guys are assuming a BTC price per chip would stay constant.  June chips wouldn't produce as much as May's and would not be worth as much BTC.  Therefore price would have to lower and dividends would be affected.  However, the USD cost is the same from month to month.  If value of BTC goes up, there should be more orders and a premium price. 

I think FC felt it was easier to give a range in USD and tie that to production costs.  But, in the end it's 6 of one and half dozen of the other.