Memory and thinking are different animals. Search the forums here for multiplication, what you describe is pretty common. Many gifted kids need something tangible to wrap their heads around and engage their minds. Rote memorization doesn't. Often times, practice is not as good as deep thinking to lock in learning. Here's a cool technique to help with multiplication tables for kids who excel at big pattern thinking:
http://www.visual-learners.com/support-files/nbf-6-9-10.pdf