Tiny piece of housekeeping, note for self for future use.
Previous reservations are wedged in the DHCP database, and not accessible through the DHCP MMS snap-in. Scope reconciliation shows the rogue entries.
Solution is to delete the reservations from the DHCP database.
Show all DHCP clients, i.e active leases and reservations
NETSH DHCP SERVER SCOPE [scope] SHOW CLIENTS
Remove individual, expired reservations
NETSH DHCP SERVER SCOPE [scope] DELETE LEASE [IP ADDRESS]
I could have done this with PowerShell, but NETSH was fast and easy.