Your coach is wrong. I gained more than "6-8lbs" and I know and worked out with people who gained more than that. That might be "average" but to hold all humans to that standard is just plain stupid. Beginners can gain more than that if they train correctly. After someone has been lifting for a while, expecting much more 10lbs of mass gain a year is pushing it. You can't just make a blanket statement for all people because your level of training, along with genetics, has much to do with it.