What's the practical difference between 96 and 99% purity?

In the first episode of season 4, Gale (in the flashback) contrasts his purity with Walt's, noting that his is 96% rather than 99%. Gus dismisses it with "How pure can pure be?" and Gale says that the difference is actually very substantial.

Why is this the case? I had assumed that the effect would simply be about three percent stronger, in which case it's not very substantial. Does the effect exponentiate as purity rises or something?

Submitted April 26, 2017 at 06:43PM by cabinboy752 http://ift.tt/2pjCTml