Lab grade is purified according to specification of highly sensitive laboratory equipment. Manufacturers of these pieces of equipment specify a purity level for the gases and this is what the lab grade is geared towards. When unwanted impurities are present, performance of laboratory instruments may be compromised, or the instrument itself may be damaged.
In our aquariums all we are concerned with is lowering the PH and we do not need a specific purity level to get that. You can pay the higher price for the lab grade co2 but your not gaining anything from it.