When breeding, it'll pick 5 random stats (since you have a parent holding destiny knot, otherwise it's 3), and for those stats, it'll randomly take the IV of one parent. For the remaining stats, the IV will be totally random.
So in your current setup, There's a 5/6 chance it takes the sp.def IV of a parent (which from both would be 30) and then the other 1/6 times, it'll be random, and so have a 1/32 chance of being 31.