Isn’t this from the custom SSD controller they’ve been iterating over for years? [0] A variation of it even made its way into the iPhone [1]. Of course there are likely more hardware factors, such as binning.

0: http://www.iphonehacks.com/2015/04/apple-custom-ssd-controll...

1: https://bgr.com/2015/09/29/iphone-6s-macbook-ssd-storage-con...