browser adds If-Modified-Since header and cache.match func (from Cache API) doesn't run

Hello! Following this basic tutorial: https://developers.cloudflare.com/workers/examples/cache-api/ When i reload a URL in an incognito tab, the browser adds If-Modified-Since header to the request. The cache is automatically hit without running the worker and executing the cache.match(cacheKey); function. Is this behaviour normal?

Cloudflare Docs

Using the Cache API · Cloudflare Workers docs

Use the Cache API to store responses in Cloudflare's cache.