The field external id's are created when the field is first made, if the name of that field is already taken then Podio will auto increment with the -1 or -2 or however many are needed. It looks like someone made that app, created quite a few fields, and then deleted them.
Since they have already been created once you can't get it to create that external id again. Maybe you could do a "one-product" for the field label, and then you can rename the field label to be what you actually want to see on the front end.
Hope this helps.