Consider a rod with two ends. Pure and Impure are two extreme ends of that rod, which acts as a reference for measurement of something which falls within those two ends. Another factor is the mindset/perspective with a view to look at things. When u say, a pure thing has 0% impurity, it is true from a mindset which has taken impurity as a reference. But well, when there is 0% impurity, then why to mention about impurity at all.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/interfaith-dialogues/511-what-does-mean-say-someone-pure.html
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=511

Hope that helps.

Regards.