See, that's my thinking, it makes so much more sense if poisoned only comes into effect if the roll would otherwise be worse. From a logical perspective it makes more sense; think of it this way, suppose I'm a Carnifex (rawr!) and I have the strength to smash through steel like it was cardboard, life is pretty good... Now the hive-mind (in it's wisdom) decides to reconstitute me with poison sacs, I STILL HAVE THE STRENGTH TO TEAR THROUGH STEEL, now it just so happens I am also poisonous, when I hit it will still be just as strong as ever, but it will also poison them.
Unfortunately, as Nabterayl points out, that isn't how the rules work, and I even looked in the errata to see if this had been addressed, to no avail...