that's called dilution. it's what happens when you like to pat yourself on the back as a good miner saying you got "all" the gold but forget to mention you picked up a few other rocks as well..
I remember several months again management was excited about the "extra " tonnes they where mining.. thought at the time I hope that's not waste rocks being presented as ore rocks..
also remember some conversations on here about the lack of grade control and geological control on the mine site...
if the head grade is F ** , it's first place you look. the second is the geological interpretation ..
(20min delay)