A pharmaceutical pure product almost always isn't fully pure, but just VERY pure. There are ratios set for the impurities which is allowed, just like e.g. USP nic which like glycerin is 99% min. Normally the EP is stricter but in the case of glycerin it's actually the other way around with "only" 98% glycerol needed. No matter what, i've never seen neither a USP and/or EP glycerin under 99.5 min, well, actually never seen under 99.6 and never seen over 99.91 or 99.93, can't remember for sure.
No matter how you slice it, the final product is arrived at through a process of fractional distillation. Fractional distillation utilizes high heat under pressure and separates substances one by one as the temperature rises. This purification process is unparalleled by any other method known to science for the purpose of separating liquids.
VG is fractionally distilled, thus it is a pure product including any residual water that may remain.
Fractional distillation - Simple English Wikipedia, the free encyclopedia
