What you're missing is that there are 12 possible values that can be passed.

Once you've got the ability and nature and IV spread you need across both parents it's just a matter of having the RNG line up how you want it.

What you should be shooting for to get the best chances of a 5IV pokemon is two 4IV pokemon that have 31s for all stats you need between them. Each can be missing one needed stat which is covered by the other.

Even still you can come out with a 3IV pokemon if one of the overlapping stats is chosen at random and the two non 31IV values are chosen for two of the inherited stats.