Why are some bars more pure than others, and is a bar of higher purity a better bar?

Since one PAX Gold (PAXG) token always equals one fine troy ounce of London Good Delivery gold, one PAXG token always refers to pure gold, regardless of the impurities that might exist in any individual bar. It is worth noting that each London Good Delivery bar has a minimum acceptable fineness (purity) of 995.0 parts per thousand fine gold (99.50% gold), which means there are always small impurities in the gold bars. However, as a PAXG holder, it does not matter if the bar has purity of 995.0, 996.0 or 999.0 as PAXG is equated to the weight of pure gold in that bar, not the gross weight of the bar overall.