As noted in Daat Mikra (quoting Mordechai Breuer's Tanakh) there is a discrepancy between the way the Ten Commandments are laid out in the Masoretic text and how they are counted. According to the common enumeration, there is a missing "parshah" break between the first and second commandments (Exodus 20:3-4), and an extra one in the middle of the tenth commandment (Exodus 20:14).
Why are the Ten Commandments not broken up in the Torah in the same way that we enumerate them?
