Requests cache by last-modified but status code is 200

I'm caching requests using Last-modified strategy.

I'm using koa and koa-static and here's part of my code:

app.use((ctx, next) => {
  return next().then(() => {
    console.log(ctx.fresh)
    if (ctx.fresh) {
      ctx.status = 304
    }
  })
})
app.use(staticRoute(path.resolve(__dirname, '../public')))

If contents have not changed, return 304.

However, I open the chrome console and found that some requests' status codes are 200, even if my server's console showing that the ctx.fresh is true.

The single request:

It didn't bring is-modified-sine header.

The interesting thing is, these requests are cached, take a look at their size, all 162B, but their status code are 200 and didn't say they're from disk-cache or memory cache.

Here's the origin size of these requests (when I disabled cache in my koa middleware).