It seems that twelve is a number with a lot of meaning in the Bible, and thirteen not so much. So why not make Paul the twelfth Apostle and just skip Matthias?
Later in Revelation the church seems to be still presented as based on twelve as a complete number:
And the wall of the city had twelve foundations, and on them were the twelve names of the twelve apostles of the Lamb. (Revelation 21:14, ESV)
So why thirteen? Or why are there twelve Apostles to Israel and only one to the rest of the world?
Note: I am not doubting that Matthias was an Apostle. This question assumes he was. I am also assuming the restricted sense of Apostle as not just 'bearers of the NT message' or 'commissioned representative of a congregation' but those inner circle of Apostles whose offices were directly appointed by Christ in the flesh, Matthias being a replacement of one of those. Therefore, I am excluding Barnabas, James, etc. from the question.
Please see also the related (but not duplicate) question Who was the 12th Apostle - Matthais or Paul? That question asked if Paul replaced Matthias, or not. This question assumes he did NOT and then asks 'What is the meaning of twelve Apostles to Israel and one to the Gentiles?' What is the meaning of 12 + 1?