You can get 100 CRIT rate, however that does not mean that you will crit 100% of the time or even that you will hit 100% of the time.
The 95% myth most likely has truth to it. Monsters have a crit shield, as with players each 5 points in monster LUK reduces your CRIT chance by 1%. Now assuming that CRIT is capped at 100, you will most likely never reach a true 100% crit chance regardless of what your stats say. With players perfect dodge comes into play as well, since P.Dodge is factored in before crits, thus even with a 100% crit chance a player can still P.Dodge your crit.
Other possibility is a similar system that flee has, flee is indeed capped at 95% in a way. Even if you have 10000 flee and you're facing a poring it will still hit you 5% of the time. No matter oyur flee the monster will always have a 5% chance of hitting you. I have no clue why it is there but there is a possibility that Crit has a similar system, no matter how high your crit is a monster will always have a 5% crit shield against it.