When you read a book with a story-in-a-story-in-a-story, you're keeping track of the nesting of the stories. (I hope.) When you have what's commonly called a dream within a dream, you aren't, and the nesting is only "discovered" post facto, after the allegedly nested dream is over. And, if my limited experience of such dreams is representative, the existence of the allegedly-nested dream has little impact on subsequent events in the "outer" dream, so there's not much psychologically or literarily to distinguish these supposedly nested dreams from merely sequential ones.

To a great extent we're arguing about words rather than facts here...