SPI fundamental question
Even if a clear specification doesn't make sense to you, you should adhere it.

If it doesn't work the way it is specified, you can start to question it.

Byte order is often different between the microcontroller and external peripherals, so be prepared to face this in the future at other points as well.

And this is nothing inherent to SPI, it's the protocol defined by the manufacturer of the chip. SPI will not determine the byte order.