Confusion about IV breeding.

I apologize if this question has already been asked, but I feel like I'm doing something wrong or I was mistaken about something.

I've got two Jormuntide Ignises (Ignisi?) who have perfect IVs. However, when I breed them together, I'm constantly getting non-perfect IV offspring.

I was under the impression that when two pals breed, the offspring has some combination of the parents' IVs, but it would stand to reason that two parents with 100's across all three IVs would create offspring with the same, since there are no differences.

I apologize if I'm just being stupid/mistaken.