I think your numerical comparison is a not...

I think your numerical comparison is a not necessarily fair.
Firstly, nobody knows what the future gold price will be so my comment about the first rule of capitalism as mentioned in my previous post applies.

Secondly, BB is saying that they chose to mine lower grade ore given the present gold price rather than "sterilize" it.
I assume that "sterilizing" effectively means abandoning that ore or at least making it unlikely to be mined given the given the mine development. 
So the choice is not whether to mine the lower grade or now or later as you posit, but rather whether to mine it at all. Mining it adds to the mine life because it is not necessarily ore that would have been counted in the mine life calculation. 
NST is saying that they choose to mine it now while they can do so at a profit. 

Thirdly, the several mentions of sequencing and the answers in the Q&A  suggests that in some cases the lower grade ore can be mined as part of the necessary development work in gaining access to the higher grade ore, thereby adding to the mine life.

I am not a mining engineer so I am happy to be corrected.

Cheers