E-transfer is not a real time payment system. The recipient's financial institution lends the money to them and then the actual funds (which were frozen by your bank to ensure money is available) gets transferred asynchronously over a period of a few days, as with any legacy bank transfer. It's just hidden away from the end-users.
This is how all real time payment systems work if the recipient institution != sender institution. Credit cards, iDeal, Bancontact, SEPA instant... In the end it's reconciled within the next few days with a pinky promise between institutions the money will actually arrive.