The tool tips say the chance to successfully reverse engineer a green item as being 20%. This is clearly rubbish. I've produced MANY MANY more greens than needed for a 1 in 5 hit rate.
I'm not sure if it's the SWTOR random number generator that's broken, or if the tool tip is wrong. Either way it should be fixed..
(and yes I know, 20% chance doesn't guarentee a 1 in 5 hit, but it should average out to that over time - and clearly, in this game it doesn't).
Each item has a 20% chance to teach you something new but also an 80% chance to do nothing at all. Every time you reverse engineer, you roll the dice again with the same odds.