I'm a hobby silversmith and this is correct. Another impurity is carbon from the torch, but the borax acts as flux. it makes sense to me.
Even if his technique is bad and he does carry impurities in, it seems that his process still guarantees you the right amount of silver. He is taking 1 oz of 92.5% and adding in .075oz of 99% shot. This guarantees .99oz of silver. If the weight is over, then the extra weight is from impurities but it doesn't matter as you have the same amount of silver.
Thanks. The weight is over primarily because I overcompensated on the shot, to ensure purity best I could. I got an inquiry about these ingots-they are SOLD, sorry. I check the marketplace everyday for someone offering scrap silver, I have some new molds and want to get back to melting!