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.
That is essentially pricing in bitcoin. If the USD price per chip increases as the exchange rate increases then it is the same as pricing in bitcoin. However, customers don't like to see their prices going up, so psychologically pricing in bitcoin is better if the exchange rate goes up.
However, if priced in bitcoin and the exchange rate goes down, AM makes less USD. Therefore, it is safer for AM to price in USD since their costs are in USD. Of course if you price in bitcoin you could increase bitcion prices if the exchange rate drops, but then you face that psychological problem again.