Why when i use my spell to identify the whoppers and then i catch them and they are keepers
The whopper and small fry identifier spells only identify the fish that are whoppers/small fry OR close to whopper and small fry...it's not a guarantee they're big/small enough, but it helps us narrow it down. It will always identify any that ARE whoppers/small fry but also identifies ones that are close to those categories too.
I hope this helps and good luck getting many whoppers!