Hi @soster,
I’m pretty sure that’s the same question I once asked about yearly snapshots: Multiple snapshots kept per year
Restic keeps the oldest snapshot as some kind of best effort approach if there are not enough snapshots for your keep rules. Your command requests to keep 7 daily snapshots but there are not enough, so it keeps the oldest snapshot to get a bit closer to the specified 7. Once you have snapshots on 7 or more days, this oldest snapshot will be removed by restic forget.
There is some more information in the link above.