两个文件描述符编号不同，为什么读取位置仍会互相影响？

"""Real ordinary-file descriptor experiment; Python standard library only."""
import json
import os
import tempfile
from pathlib import Path


def experiment():
    checks = []
    observed = {}

    def check(name, actual, expected):
        if actual != expected:
            raise AssertionError(f"{name}: {actual!r} != {expected!r}")
        checks.append(name)
        observed[name] = actual

    with tempfile.TemporaryDirectory(prefix="dup-offset-") as folder:
        path = Path(folder) / "letters.bin"
        path.write_bytes(b"ABCDEF")
        descriptors = {}
        flags = os.O_RDONLY | getattr(os, "O_BINARY", 0)
        try:
            descriptors["A"] = os.open(path, flags)
            descriptors["B"] = os.dup(descriptors["A"])
            descriptors["C"] = os.open(path, flags)
            a, b, c = (descriptors[key] for key in ("A", "B", "C"))
            check("three_different_numbers", len({a, b, c}), 3)
            check("A_read_2", os.read(a, 2).decode("ascii"), "AB")
            check("B_position_after_A_read", os.lseek(b, 0, os.SEEK_CUR), 2)
            check("B_read_2", os.read(b, 2).decode("ascii"), "CD")
            check("C_read_2", os.read(c, 2).decode("ascii"), "AB")
            check("A_position_after_B_read", os.lseek(a, 0, os.SEEK_CUR), 4)
            check("B_seek_start", os.lseek(b, 0, os.SEEK_SET), 0)
            check("A_read_after_B_seek", os.read(a, 2).decode("ascii"), "AB")
            check("C_position_unchanged", os.lseek(c, 0, os.SEEK_CUR), 2)
            os.close(descriptors.pop("A"))
            check("B_read_after_A_close", os.read(b, 2).decode("ascii"), "CD")
            check("B_read_remaining", os.read(b, 10).decode("ascii"), "EF")
            check("B_at_end", os.read(b, 1).decode("ascii"), "")
            check("C_read_independently", os.read(c, 2).decode("ascii"), "CD")
            check("file_bytes_unchanged", path.read_bytes().decode("ascii"), "ABCDEF")
        finally:
            for descriptor in descriptors.values():
                os.close(descriptor)
    return {"experiment": "real ordinary-file os.open/os.dup/os.read/os.lseek",
            "checks_passed": len(checks), "observed": observed}


if __name__ == "__main__":
    print(json.dumps(experiment(), indent=2, ensure_ascii=False))