Why was
a. africanus unlike any primate fossil skull that dart had ever seen?

The A. Africanus is an odd ape-like creature possessing human traits on the teeth and eye orbits, specially on the base of the skull over the spinal column which had a hole, foramen magnum as it is called. The foramen magnum is in placed similarly to human upright posture implying A. Africanus is bipedal rather than quadrupedal. This was the first time "ape" was added to hominins declaring that humans descended from apes.