There's a lot of debate between the 'true' Space Grey since it varies so much, the difference between the iPhone and the macbook has divided opinions on here.
I think space gray was created because scratches showed so much on the black iPhone 5. The closer the color is to the natural silver color of aluminum, the less the scratches show up. They definitely went a shade lighter on my Space Gray iPhone 6s vs. my 5s. I think Apple decided that since a MacBook doesn't get handled as much as a phone, they could use a darker shade of gray.