I would assume that the interest would be net so would have to take into account what they are earning on the cash and environmental bond (which I would assume to be funds in escrow that should still earn the company a return but am not sure on this).
Having said that, I agree that this wouldn't explain the difference as they would have to be earning over 10% on the cash and environmental bond in order to earn the $30 per ounce difference, and this doesn't seem plausible at current market rates.
Hopefully all will be explained in the half yearly figures in January.
NGF Price at posting:
21.0¢ Sentiment: LT Buy Disclosure: Held