They’ve each got half the brain

Though their bodies are unchanged from how they should be, Hermione and Andromache’s brains are not copies of what they were. The matter which made up the original brain is conserved, and split equally between the two twins. Because of this, Hermione isn’t as bright as she once was, and neither is Andromache.

With each only bearing a brain half the size they should, they’ve both got an iq of half Hermione’s original 138: 69!