Maybe Im not thinking of this right? Idk
Finished oils contains lets say MCT(most common) BB BA.
Lets fix BA at 1.5%. Then we play with MCT/BB. If its low purity, we just bump up the BB% and lower the MCT%, which allows more of the "stuff" to dissolve, till we hit the desired mg/ml. We then get a finished product with the desired mg/ml, which also contains impurities.
100% purity @ 100g gives you 100, 50% purity @ 200g will also give you 100. Its all just math and numbers. and adjusting MCT/BB ratios.
Sure at the end you filter it, but what you filter is stuff that has not dissolved, aka residue.