When Apple installs two 512 GB chips in one of their computers for a total of 1 TB, the computer reads and writes to both chips simultaneously, increasing the read/write speed. But when you purchase the 512 GB model it will be slower because there's only one chip to read/write to. This is a natural part of selling a computer, so it's not Apple's fault. Apple could have installed two 256 GB chips for faster speeds, but then they would need to purchase an inventory of 256 GB chips for only one configuration (512 GB model). Instead, Apple purchases 512 GB chips, which are then suitable for two configurations: 512 GB, and 1 TB. This saves Apple from having to purchase and store an inventory of 256 GB chips.