It amounts to knowledge by the AS whether this is a confidential or public client. When registering a native app, Google knows that it can only be a public app. When registering a web app, they can assume this will be a confidential client and expect you to keep the secret, well, secret. The AS (Google) can then have different policies regarding what scopes they allow, or how they present the consent screen and admin panel, depending on the type of client.
Have a look at the definition of both types of clients in RFC 6749.
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