By default Levels clamps input luma to [16,235], converts it to [0,255] for making changes, and then scales the output back to [16,235].
As such, Levels(0,1,255,0,255), does nothing on a [16,235] clip, but it clamps (or rounds) a [0,255] clip to [16,235].
http://avisynth.org/mediawiki/Levels
Don't let the numbers confuse you ChainsawAsh, the filter works like it should.