Those margin values come from the template you have chosen while creating that Pages document. If you wish to have some of your new Pages documents to have different margins by default, the only solution would be to create and use your own templates because you can't delete or modify itself any of the templates that come with Pages.

To create your own template with the margins you prefer: 

 1. Create a document from any of the existing templates in Pages whose characteristics are already close to the template you wish to create and change its characteristics including the margins (or duplicate one of your existing Pages documents with the closest characteristics, and empty and change it) as you like.
 2. Go to File->Save As Template in the Pages menu bar and from the menu to come up, select Add to Template.
 3. Your new template will be shown under My Templates in the window to appear. Name it as you like and your new template is created. You can then select Cancel or, if you need a new document with those characteristics right away, select Create.

To use an existing template of yours:

 1. Select "New Document" in the window that comes up right after Pages is launched the first time or press Command+N if Pages has already been launched.
 2. Go to My Templates at the bottom on the left margin of the "Choose a Template" window, double-click on your template or just select your template and select Create at the bottom right. After you use that template at least once, it will start appearing at the top of the "Choose a Template" window under Recents and you won't have to go to My Templates.

Through the "Choose a Template" window mentioned above, you can rename or delete any of the templates you have created. Just find your template and right-click on it for either.

For more, see the "[Create and use page templates in Pages on Mac](https://support.apple.com/guide/pages/page-templates-tan1959e234e/12.2/mac/1.0)" Apple support webpage.