Seed Phrase in Electum

[Yamane_Keto]

But if you have a BIP39 seed phrase and change any of the letters to an upper-case letter, you will see a message saying "BIP39 (unknown wordlist)" and you will get completely different addresses.

OP generated his/her seed phrase on another wallet and it was BIP39.

But why isn't the principle the same as the seed phrase which is rechecked using the BIP 39 dictionary list, if the phrase exists it will give you guesses for the word, otherwise you will get an unknown wordlist error message?