I have already mentioned this in another thread and I didn't realize there was a specific section for this error on the site until now.

http://www.psypokes.com/hgss/breeding.php

On the page regarding Power Items in Breeding, it mentions, "If both the mother and father are holding a Power EV item, then two IVs will be inherited..." This isn't a hard fast rule. While it is POSSIBLE to get both IVs passed on, it is not guaranteed. If both parents have a power item, one of the two stats will be passed on randomly between the two (I'm getting about a 50/50 split between the genders), then normal IV inheritance rules apply.

I have the research to back this up (I had doubters on the other thread). If you would like to test this yourself, get two pokemon with different stats (preferably low IVs) except for each one having 31 in a particular stat. Give the power item to each parent in the 31 stat. Now breed. Now, the easy detection is by looking at their characteristics, and you'll see that unlike if you give one power item and the characteristic is (almost) always the max stat of the power item, it will of course be swapped 50/50 between the two 31s. Now, this isn't a sure thing because everyone should know if you have two 31s the characteristics are random between the 31 stats. You will have to level up the pokemon (generally about level 10, if you are recalculating the IVs between each level) you should be able to pinpoint that some of the pokemon will NOT have 31 in BOTH stats. They will have inherited a stat, (say you have a Bracer on one of the parents, then Attack) either from the other parent, or a random # is generated). Since this is determined by the normal inheritance rules and not because of the power items this is why it is possible to get both IVs transferred onto the baby where the confusion might have come from. The odds of passing on multiple IVs is increase when both parents have the max stat, up to 3 (but this is no different than normal inheritance rules). I have not yet gotten to the point to determine if the power items replace one of the 3 stats inherited, or if it is added before normal inheritance. If the later is true, the odds of passing 4 IVs to a baby would be increased dramatically.