Typically, when you have an app shortcut icon added automatically to the homescreen, it'll be the first homescreen where there's an empty spot at the lower right of the screen. If there's already an icon there (even if the rest of the screen is empty), the system will often default to the next homescreen to the right.
Remember also that "homescreen" refers to any of the homescreens. Your primary homescreen is the one you set as the default one (typically by going to the App Overview, selecting your primary homescreen, and tapping the Home icon above it). So the option to add a shortcut icon automatically to a homescreen doesn't specifically mean your primary homescreen.