Magic and miracles are two sides of the same coin in The Lion, the Witch, and the Wardrobe, and it can often be difficult to tell where a spiritual miracle ends and a magical happening begins. In order to convey spiritual ideas to a child audience, fantastic and supernatural tropes are used. Magic may be good or evil, or may simply be a fundamental structuring principle for the world. Magic doesn't always work in the same way twice, and miracles may be more complicated and painful than expected.
By describing Aslan's resurrection as magic instead of a miracle, C.S. Lewis focuses on the fantasy-adventure dimension of the story, rather than the religious doctrine underlying it.
C.S. Lewis chooses to describe Aslan's abilities as magic, rather than as miracles, because magic is an integral part of the world of Narnia and has clear, inviolable rules, while miracles imply an interruption or exception to a world's rules.