为什么使用线程的脚本偶尔会打印额外的行？

import threading

class ThreadSafePrinter:
    def __init__(self):
        self.lock = threading.Lock()

    def print_line(self, line):
        with self.lock:
            print(line)

printer = ThreadSafePrinter()

def print_lines():
    for i in range(10):
        printer.print_line(f"Line {i}")

threads = [threading.Thread(target=print_lines) for _ in range(5)]

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()