A simple hard drive today is capable of things that sound like some outlandish technology. Just try to do some file I/O in your application and do it with many threads.
Say you have 4 threads, A,B,C, and D. And request to do I/O comes in A then B and so on. If you check the return status of these threads, the ordering might be surprising. Thread D may return before A. How?
Disk have a technology called Native Command Ordering. So they take your request in and process them on a single, simple logic:
-> Serve the one which you can do fastest.
This depends on the head position of the disk. The request that can be served with minimal movement of head, is served first.