Thus some books attributed to Paul, but that reflect a different style/context/content are likely written by those who studied with/under him. The early Fathers chose those manuscripts to include in the New Testament that were written by Apostles or those who were close to Apostles and that included all the writings with Paul's name on them.
Which is why I said in an earlier post that Paul is not only given enough credit, but was the most honored person who authored any of the New Testament in what we now call The Holy Bible.