Why is Amazon a merchant and not a spending category?

A category is the type of spending — groceries, utilities, software, travel. A merchant is who you paid — Amazon, Costco, Shell. Amazon sells across many categories, so your Amazon spending naturally spreads across several of them. That's why the Spending Breakdown gives you both views: "by category" answers what you spend on, and "by merchant" answers where your money goes — including a single "Amazon" line totalling everything you spent there.