Since you deny any other variant as impossible or very low probability, settle for this thing you said yourself.  If there is a possibility you generated it outside electrum, think which wallets you used.  If I remember properly, wallets like Wasabi Wallet use for example different derivation paths when compared to Electrum.  This does not mean YOU have done something but the wallet may have.

Maybe it is wise to look through lists of wallets and see whether you recognize any of them as one you have used in the past.  Give it a try, at least.  If you recognize one, try the seed in it again.  See if it shows your balance.  But make sure you install the legitimate wallet, verify the files before installing them.  This is so that you do not get scammed out of your wallet by installing malware.

If you have ever used hardware wallets, it is the same situation.  For example, Ledger and Trezor use different derivation paths and I believe you get a different set of addresses if you import the seed phrase of a Trezor in to Electrum with out setting the right derivation path.

Also.  If the two wallet addresses start with bc1q, they are definitely not Legacy.