Why do some ebooks say “Instant download” and others say “Kindle sync”?

Every paperback ships through Amazon, since they handle printing and shipping better than we could on our own. Ebooks vary by title: some are sold directly through us and land in your inbox as an EPUB/PDF within a minute of checkout; others are sold as Kindle editions through Amazon and sync to your existing devices. Each format button says exactly which one you’re getting.