hmm.. ok.. how come?
Let's take a look at the specs:

Your MacBook Air uses an Intel i5-5250U processor. It contains 2 CPU-cores and the GPU Iris 6100.
The 2 CPU-cores are highly complex and powerful and clock between 1.6 to 2.7 GHz.
The Iris 6100 contains 48 GPU-cores, which are way more simple than the two CPU-Cores. These 48 cores clock at 300 to 950MHz.

So you have two complex CPU-cores at high speed vs. 48 less-complex GPU-cores at lower speed. Let's compare the figures you measured:

CPU: 51787 kH/s -> divided by 2, each core does 25894 kH/s
GPU: 427900 kH/s -> divided by 48, each core does 8914 kH/s

See the difference? More cores are better than a few. At least most of the time.

Take a look at your desktop GPU. The Radeon 5670 comes with 400 cores. The ones Royce postet have 640 and 1664. High-end cards go up ~3500-4000. Every CPU with 8, 16 or even 32 cores is standing no chance. In Hashcat, only a few and very complex hash functions prefer the CPU. This rest is lightyears faster with the GPU.