No problem.
So it sounds like just a misunderstanding of what that button is on your side, when you create an account with an email address that's what it is and it can be any email provider.
When you click the Google button, that's creating what we call a "Google social network account". This uses some behind the scenes magic with your Google account, rather than your Gmail address.
An easy way to understand might be the fact we also have Facebook logins available, while we don't have a login button for Outlook emails.
Also worth saying and being clear, you haven't created a jagex account if you're logging in with Google. You may have linked a Google account to an existing RuneScape character which itself is linked to a Jagex account, but to get the full jagex account benefits you should log in with an email address and MFA.
Is that clearer?