Well, imagine the Day Care as an IV compounder, or mixer. You can't put spinach and carrots into a mixer and expect apple juice, can you?
In a similar fashion, the offsprings can only inherit the IVs that they're parents have. The two Pinsirs have good Attack and HP IVs, so those would be the IVs that pass down to the offspring. On the other hand, good IVs like Defense or Speed which are not present in the parents would not be inherited. Those would be decided by luck, as usual.
To get the Pinsir you want, you would have to switch around parents a bit. Before we proceed, please place all your ethics aside in that box, because GF gets weird starting now.
First, you must somehow manage to get a 6 IV Ditto (or a 6 IV male Pokemon from the Bug Egg Group). Try asking some people to lend one to you. Secondly, procure a female offspring with the HP and Attack IVs. Commence the breeding of the aforementioned Pokemon, making one (any one) hold the Destiny Knot.
- The first generation of offsprings will mostly have 2 IVs, mono IVs and sometimes 3 IVs. Make sure you choose a 3 IV one with the IVs you want, and breed it to the Ditto
- The second generation would produce greater results. You'd still get 2 IVs, but the chances for 3 IVs and 4 IVs increases. Take the 4 IV one (again, make sure it has the IVs you want) and breed it to the Ditto
- The final generation would produce mostly 3 IVs, some 4 IVs, and on rare occasions, 5 IVs. The 5 IVs one is your target, and you have to pry on this chance to get your perfect Pinsir.
And that was the easiest method to get a 5 IV Pokemon. Basically, it's a lot of breeding, egg hatching and disappointment. Don't lose patience though, eventually the 5 IV would emerge.
Hope I helped!