To answer your question, you first have to understand how evasion rate works.
Basically you have two things to determine how much you evade. Your cumulative evasion (skill + gear) and the mob's accuracy. No matter what, there is a floor and a cap on evasion rate.
So for example: a lvl 90 THF having a lvl 1 bunny on them will of course have capped evasion from skill alone. Adding any more evasion gear won't help him evade any more. At the same time, that level 1 bunny can still hit the THF because of the lower cap on evasion. It may take a while, but still possible. Much higher level mobs will have more accuracy, therefore uncapping the THF's evasion. Which is where you would add more gear.
Think that's pretty much the simplest explanation that I can give you.
Check this link
if you want to read a more detailed explanation on hit rates. Might help you understand the concept better.