Bill pay within Fidelity sent as paper check

I read that I can use bill pay as an alternative to transfer funds between accounts. I gave it a try, using Fidelity Bill Pay, added a payee “Fidelity Investments” with one of my Fidelity account number.

When I did an initial test by bill pay $1, it completed the 2nd day. Looks that must be done electronically.

However I then setup reoccurring monthly pay $6000, the payment method has been snail “paper check”.

Why is this? Why bill pay to Fidelity account doesn’t use electronic method / EFT?