Your document is set to facing pages or not.
If you have "facing pages" switched on, you will see each spread as it will be printed — even pages on the left, odd pages on the right. If you switch off facing pages, you will always see a single page, regardless of its page number.
You can see the difference even when your document only has a single page; a non-facing document will display it in the center of the screen, a facing document will show it on the left or right side.