There are many docking systems on the International Space Station, reflecting the fact that it's the product of multiple space programs which combined their space station plans into the ISS. The history of the program has lead to some design choices which seem to be strange, until you look at them in the context of the whole program history.
In particular, I often get asked about the pressurized mating adapters at the front of the space station and how the tunnel includes a bend rather than simply going straight through, and of course it's all because of historical choices.

Some further reading on the docking and berthing hardware used on the ISS
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf