We have 15 prophets, seers, and revelators in the Church today (a First Presidency and the 12 Apostles). I do not see that a First Presidency existed in the time of Christ although we commonly accept Peter, James, and John were the acting presidency of the time after Christ had ascended. But that only leaves 9 other apostles. Why the different organization?
When Christ was on the earth there was no need for a First Presidency for he was in charge of things and his own spokesperson. It was only after his Ascension that the role of the First Presidency became necessary/vacant. And like it happens today when there is a vacancy the senior apostle became the leader and spoke person. This creates opening(s) in the 12 which are then replaced. After all it wasn’t that long before after Christ left, that Paul joined the Twelve.