Agreed ! It seems that Paul used the word of himself in a new way, and later Writers like "Luke" picked up on the idea, and invented the Apostles of the Christian Movement.
K. Lake in the Books "Christian Beginnings", writing in the 1930's observed :
" Thus we have the extremely interesting linguistic fact that a Pauline-Lucan branch of Christian literature seems to have popularized and given a technical meaning to a word which was otherwise scarcely used, except in a different sense, in the whole course of previous Greek literature".