The "Matches Natural Search Detection Rules" depends on Adobe Analytics being able to recognise a referrer as a search engine. You can read more about this recognition at https://experienceleague.adobe.com/docs/analytics/components/dimensions/search-engine.html?lang=en.
Some traffic could be coming from Google properties that are not the search engine, e.g. Google Images, Google My Business, Google Maps, etc. In such cases, Adobe Analytics would classify them as Referrers instead of Search Engines.
If you want to recognise all Google referral traffic as Natural Search, then you should set up another Processing Rule for your "Natural Search" Marketing Channel (yes, you can have more than one Processing Rule per Marketing Channel) and configure the rule to include all referrals from Google domains. Then order this Processing Rule to come before your existing "Matches Natural Search Engine Detection Rules" Processing Rule.