I have been reverse-engineering the Z-80 processor using images and data from the Visual 6502 team. The image below is a photograph of the Z-80 die. Around the outside of the chip are the pads that connect to the external pins. (The die photo is rotated 180° compared to the datasheet pinout, if you try to match up the pins.) At the right are the 8 data pins for the Z-80’s 8-bit data bus in a strange order.
The motivation behind splitting the data bus is to allow the chip to perform activities in parallel. For instance an instruction can be read from the data pins into the instruction logic at the same time that data is being copied between the ALU and registers. The partitioned data bus is described briefly in the Z-80 oral history, but doesn’t appear in architecture diagrams.
The complex structure of the data buses is closely connected to the ordering of the data pins.