How does the Destiny Knot work when breeding in Pokemon 6th Generation?

Does the Destiny Knot guarantee that 5 IVs, ramdonly selected from the parents, will be passed down to the egg?

I have both parents with 4 maxed IVs. One has HP, Atk, Def and SpDef; the other one has Atk, Def, SpDef and Speed. One of them is holding the Destiny Knot, but I'm still getting Pokemon with "decent potential overall" and only 2 or 3 maxed IVs.

How is this possible?

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3 Answers

Building On @Batophobia's answer, let's look at the probabilities that occur when 5 stats are transferred.Getting either 2 or 3 Perfect IVs on the child will occur in roughly 56.9% of all eggs (where the Destiny Knot transferred 5 stats). If the IV checker is reporting 'decent' potential, then it would seem likely that whatever stats the two parents have that aren't 31 are quite low.

From the wiki it is not a guarantee.

Secondary effect:
If held by a Pokémon in Day Care, there is a high chance that five of the parents' IVs will be the same on the child.

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Destiny Knot means five IVs are guaranteed to pass down from between the parents, however, which IVs is totally random, and sometimes those IVs can overlap. In other words, say stats HP, Defense and Speed pass from parent 1, and Defense and Sp Attack pass from Parent 2. Defense has already passed from Parent 1, so, Parent 2's Defense replaces that of Parent 1. If Parent one had a Defense stat of 31, and Parent 2 had a defense of 10, child gets the 10 Defense

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