Short answer: Electrum seed phrases and BIP39 seed phrases differ in the way they handle passphrases. Since you are entering the seed phrase as a BIP39 phrase, you use the BIP39 method, which uses the word "mnemonic" rather than the word "electrum.

Longer answer: When your seed phrase is used to generate your private keys, the first step is to pass it through a key stretching function called PBKDF2. The PBKDF2 function used has two input parameters. The first input parameter is your seed phrase. The second input parameter for BIP39 seeds is the word "mnemonic" concatenated with your passphrase. You can see this in lines 810 and 811 of the Electrum code here: https://github.com/spesmilo/electrum/blob/9d0bb295e6f55a2bff9f5b6770fa744c16af6e8a/electrum/keystore.py#L810.

Conversely, when using Electrum seeds, instead of using the word "mnemonic", it instead uses the word "electrum" concatenated with your passphrase. See line 164 here: https://github.com/spesmilo/electrum/blob/a0b096dcb2292c2826f7beae173c529d335142f0/electrum/mnemonic.py#L164.

Even although your seed was generated by Electrum, since we are using it as an (invalid) BIP39 seed we need to follow the BIP39 method. This will hold true even if you don't use a passphrase, as the word "mnemonic" will still be used as an input for PBKDF2, just without any additional characters attached.