I have noticed that the sensor royalmail_deliveriestoday goes to a state of "unavailable" ... potentially after a delivery to my house of a tracked item that was not in my tracking list.
I will try to monitor this in the coming weeks.
It would help to know where the "mail pieces" data come from? is it the tracking app or some other area of the account?

If I restart home assistant it correctly re-calculates to 0
I also seem to have one "mail piece" shown but don't know what it is ..
checking my online account, in the tracking section, I see three items shown, 2 marked as "In Transit" (but are from February this year) and one marked as "Sorry, we're not able to retrieve tracking information for this item" which I suspect is old.
Is it possible to add more detail, via attributes, to the mail pieces entity? maybe the tracking ID ?...