What coverage means

Coverage is not a quality score — it's a measurement. If a profile reports 12,400 followers and a snapshot captured 500 of them, the coverage badge reads 4% and shows both numbers: "Indexed 500 of 12,400 — Coverage 4%". Never 12,400 as if 500 were the whole set.

The diff engine — the thing that decides "these accounts followed" and "these accounts unfollowed" between two snapshots — refuses to run when either snapshot is below 99.5% coverage. Below that threshold the missing accounts could easily be members the crawl just didn't reach, not real unfollows, so the honest answer is "comparison unavailable", not a fabricated list. The same rule applies to Follower Compare and to Saved Searches.