Because that's very possible with WP. Many themes do that because it allows you to set a custom template for your homepage. (Usually multiple styles of blogs.. maybe wide photo, or small photo, or no photo.. just many different layouts).
From there it just adds pagination normally.
In that page template however, they do the post loop I guess? I'm not really a WP developer, but I do know many themes Ive used in the past allow you to do this.