Far as I can tell it is just a 20% of something happening. Not a 20% chance of happening, there for after 5 times it should = 100% sort of thing. Each time it is just a straight up 20% chance.
That and RNG hates some people more then others
RNG is also temperamental like my 4 yr old son.
Case in point, My Armormech had Re'd 24 (i kept the 25th one cuz it was augmented & I needed to upgrade my gear) Overkill Heavy Exoskeleton Belts
w/o getting a single purple schematic. However, she RE'd 5 Overkill Heavy Exoskeleton Gloves
& got 2 Purples from it.