When you create a periodic snapshot task, then change something later, the task seems to "forget" that any of the prior ZFS snapshots exist and they end up remaining on the server forever (or until they are manually deleted). I've had about a dozen tickets on this issue in the last 2 months or so.
I reproduced this on 11.3-U5, and if you create a periodic snapshot task and let it run, it'll do its own cleanup and "just work". But if I change anything (in my test environment I changed the name of the snapshots that would be generated and changed the retention time) then the old snapshots are immediately "ignored" by the middleware after that. For many customer this means dozens or hundreds of snapshots and they open a support ticket for assistance as they don't want to click a checkbox 100s of times to remove them in the WebGUI.