SA doesn't remove the possibility of failure, it just increases the odds of success. At least to a point. With an SA of 458, you will get +45% success rate on all your Missions, but that won't bump you above your 95% success possibility.
That is, if Mission A had a success probability of 23% and Mission B had a success probability of 85%, your SA would adjust them to 68% and 95% respectively.
That adjusted success probability is what is displayed on the mission board itself.
I can guarantee that the code doesn't have any hidden gotchas in it, it's truly random. Random only looks like it isn't when things go poorly.
Cheers
Mike