When you go to the Abandoned Cart menu, you will find three sections with lists,
This section shows how many carts have been abandoned and are recoverable. This means that if someone has added something to their cart but didn’t complete the purchase, then they will be added to this list.
However, they should be logged into their account or at least provide their email address on the checkout page.
This section shows how many carts have been recovered from the email campaigns you ran. So basically, it will show the list of the people you have successfully brought back.
This section shows the number of carts that have been lost. This means that the recovery period is over and there is no way to recover that cart.