A little history might help.
The first version of zynthian had a combination of an external chip to handle some of the encoder pins and then used RPi GPIO pin for others. this produced the GPIO settings which were a low integer including zero that matched the pins chosen. I don’t know why it was done that way, perhaps @jofe remembers…
People quickly moved on to pure GPIO pins which worked quite well and is heavily documented in the pre dawn history of this forum.
The move to an i2c based MCP23017 came in with a desire to simplify the process and use far less pins on the Pi. and allow extra connections like the A to D and D to A pins and i/0 switches, where we would have simply run out of available pins. This also figured because the Pi itself doesn’t have any analog ins or outs among its GPIO connector which means something had to be constructed for this. and so the addressing number moved into the hundreds as a mechanism to make the interfaces consistent.