Long story short, I am breeding Deino to inherite S.Def and SPD with 31IV.

But 1 of them baby having S.Atk and SPD with 31IV. Am I missing something?

If  Im not really sure what your talking about so and posted something irrelevant and since I cant delete my post I have this paragraph so it isn't just empty... >.>

First off, I don't exactly understand what you typed.

Second of all, I THINK you're asking about why its gaining SP ATK instead of SP DEF.  Quick explination of breeding.  The male passes on the traits (moves) while the female denotes the Pokemons race (male Rhydon mating with a female Tyranitar will always produce Tyranitar babies).  However, its a total random thing how the Pokemon will still turn out in terms of IVs.  One time I mated a male Butterfree with IV SPD of 28 and a Female with SP DEF of 31 and the resulting baby had IV 31 of both stats.

This is why Pokemon breeding is annyoing.  Because you have to mate literally hundreds of babies to get the right IVs, EVs and personality.  I hope you enjoy riding that bike.

Basically, I equiped the parents with the Power items that "force" the stat of choice to be inherited.

I hatched about 10 eggs and checked the IV of several. Out of the ones I check, they all inherited 31IV for SDef and SPD correctly except that one.

I always thougth IVs were completely random...

You can customize them a bit. If you equip everstone, for example. You get 50% chance of getting same nature as the parent holding the stone.