The first scan is the starting point
FollowerSignal cannot recover follower history from before it observed the account. The first completed scan saves a baseline in this browser. A later scan compares the same username with that saved list.
Browser data can be cleared
The public-beta baseline is device-local. Clearing site data, switching browsers, or using another device can remove that local history. Account-based scheduled reports use separate server-side records when that paid feature is active.