Companies use projections for how well the line up will perform when they refresh it. They might be optimizing for different factors so the underlying logic is not always obvious but you can get a fair idea about where they are coming from if you visit a big box retailer during a weekend and just watch the sales interactions that are taking place. The 'average' consumer's needs are vastly different from the HN crowd, and even among such 'average' consumers preferences can vary widely by geography (primarily because of the difference in income) - something that companies other than Apple try to cover with product lines.