Or... it's a lot different because Paul did not write it.The book of Hebrews is a little different because Paul was writing to Hebrew Christians and not Gentile Christians.
So are you retracting your statement that the twelve "learned the gospel on the day of Pentecost"?Paul is the only apostle that had the knowledge of the covenants, the Gospel and justification by faith and was a scholar of the law.
They WERE unlearned, but Christ taught them for YEARS.Most of the other apostles were unlearned and had little knowledge of anything.
None of this is proof that Paul wrote it. The truth is that we do NOT know who wrote it.Much of what Paul said can be found in his other epistles and in the Old Testament, which he frequently quoted.
Regardless of who wrote it, it contains doctrine related to God's relationship with Israel. It is different from the 13 epistles that we know that Paul wrote.