Nobody seems to know. Dodge-rolling, for any player, grants a "dodge-period" of less than a second where every single-target skill fired at you during this dodge-period will miss. The way I understood it was that shuffle/blur gave you a 20% chance to proc the same "dodge-period" you get from a dodge-roll. This would explain how so much can miss a shuffle user cos with 20% proc chance it is quite likely you will get more than one proc in a row, essentially extending the "dodge-period".

@Alcast originally put me on to this explanation, but I've since been told it's wrong by nobody with authority. Who knows?

Major Evasion seems to work a bit differently than dodge rolling in my experience. With Major Evasion, dodging an attack is on an individual attack basis. So, the chance to dodge each individual attack is separate. I could get hit by enemy A and dodge enemy B, even if they attacked simultaneously. The dodge rolling as described above seems to be accurate, though, in that there is a very brief period after a dodge roll where everything misses.