单例模式在多线程和多进程环境下为何表现不同？

import multiprocessing
import threading
import time

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

@singleton
class SingletonClass:
    count = 0
    def __init__(self):
        SingletonClass.count += 1

def worker(name):
    for _ in range(100):
        instance = SingletonClass()
        instance.count += 1
        time.sleep(0.01)
        print(f"{name}: count = {SingletonClass.count}, id = {id(instance)}")

if __name__ == '__main__':
    # 多线程测试
    threads = [threading.Thread(target=worker, args=(f"Thread-{i}",)) for i in range(2)]
    for thread in threads:
        thread.start()
    for thread in threads:
        thread.join()

    # 多进程测试 (注释掉多线程部分后运行)
    # processes = [multiprocessing.Process(target=worker, args=(f"Process-{i}",)) for i in range(2)]
    # for process in processes:
    #     process.start()
    # for process in processes:
    #     process.join()