a lot of scenarios can change between the time you get the drill grade and mining grade.....here is an example......suppose you intersect 3 feet of au grading 1 oz......the minimum mining width for a human being is basically shoulder width plus a couple of feet on either side....what this ends up being is 7 feet grading [{(3 times 1) plus (4 times 0)}divided by 7] 0.43 oz......if during blasting you end up with more than 7 feet say 8 feet again the grade would be reduced to o.38....in addition gold vein grades may not be continuous in all three dimensions.....something similar to the nugget effect....i know this is very simplistic but i hope it helps