Assuming Affable Geek is right about the Greek, then that would largely explain it. One thing about English, unlike ancient Greek (and many other languages, like Spanish which I know a lot of), is that in English, we use the same word for the second-person singular and second-person plural. In other words, we say "you" whether addressing one person or many. In Greek, it's clear what is being said (if it is you or y'all).
Regarding the apostleship, it is interesting that originally there were 12, and then one was replaced, but by 2, sort of; do you know what that sounds like? The 12 tribes of Israel! There were 12 sons of Israel, yet one of them, Joseph, gave his lot to his 2 sons. In reality, the "twelve" tribes are, for all intents and purposes, 13 (though they are considered twelve because each of Joseph's sons got half a share). Yet the Old and New Testaments consistently refer to the 12 tribes of Israel (except when discussing them in technical matters, because God can count, lol). This is complicated all the more by the fact that Revelation 7:8 refers to the tribe of Joseph, then refers to the tribe of Manasseh (one of Joseph's sons) who had a half-share in Jacob's blessing, and then leaves out Dan (one of Jacob's 12 sons) and Ephraim (Joseph's other son) completely! There are all kinds of theories as to why this is, but whatever the case, even the tribes themselves are treated with a great deal of fluidity, just like with the apostles.
With that said, as to where Paul and Matthias come into play with the judging of the 12 tribes, I'm not sure it is 100% clear; my guess would be that the 13 collectively judge the 12 tribes, which in itself is an idea not clearly described in the Bible, so that's just speculation on my part.