Svelte store的subscribe()为何在值未变更时触发？

问题背景 #

我定义了一个Svelte的可写Store activeAccountIndexStore，代码如下：

export const activeAccountIndexStore: Writable<number | "native" | null> = writable(null);

它的初始值是null，在完成部分网络请求后，会被设置为具体数字或字符串'native'。

同时我封装了一个订阅方法，供其他组件调用：

export const onChangeActiveAccount = (activeAccountHandler: ActiveAccountHandler) => {
  activeAccountIndexStore.subscribe((newValue) => {
    log(`➡️➡️➡️➡️➡️ activeAccountIndexStore new value`, newValue);
    if (newValue !== null) {
      const activeAccount = getActiveAccount();
      activeAccountHandler(activeAccount);
    }
  });
};

异常现象 #

实际运行中发现，即使Store的值没有发生变化，订阅的处理函数仍会被重复调用，日志输出如下：

stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value null
stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value null
stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value 0
stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value 0
stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value 0
stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value 0
stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value 0

MDN的Svelte文档说明：

A store is an object with a subscribe() method that allows interested parties to be notified whenever the store value changes.

按照文档描述，只有当Store的值发生变化时才会触发订阅通知，但现在的情况明显不符合预期。我原本预期的日志应该是：

stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value null
stores.ts:68 ➡️➡️➡️➡️➡️ activeAccountIndexStore new value 0

疑问 #

为什么Svelte Store的subscribe()会在值未变更时重复触发？

内容的提问来源于stack exchange，提问作者mikemaccana