Wallet migration leftovers
When migrating funds from a Byron wallet to a Shelley wallet, on some occasions, depending on a combination of transaction inputs and outputs, a very small amount of ada cannot be transferred due to a Cardano protocol restriction of 1 ADA as a minimum transaction output.
Although the migration tries to preserve the UTxO shape of a wallet, it has to sometimes resort to combining some small UTxOs together but there are still some rare cases where it could fail combining many dust coins to a value that is big enough.
This ada is a leftover that will remain in the source wallet. Daedalus displays the amount leftover in the source wallet when transferring funds.