Toz had only relatively recently acquired the void and space elements. But since he used void magic every time he used combination magic, he was getting quite proficient in the attribute.
However, Toz's proficiency in the void element was mostly limited to when he used it in combination with the other elements. It didn't help much when he tried to learn Dissolution, a spell digging into the true nature of void magic.
Most of Toz's experience with void magic was pretty useless when he tried to learn the spell. Since the Void inspired his darkness spell, Void's Embrace, it could provide some guidance. But it was inspired by the Void's properties of omnipresence, all-encompassing, and containment, so it couldn't help Toz much, not to mention how it was still a darkness spell and built on the nature of darkness magic.
