Classic (day-N) retention counts users active exactly on day N; rolling (or unbounded) retention counts users active on day N or later. Know which one you are reading, as they are not comparable.
D1 is a promise-keeping metric — did the first session deliver what the store page and ad promised? D7 and D30 reflect the core loop and meta-progression. Because retention curves follow a steep early decline and a long tail, small changes early compound into large differences in lifetime value.