Well, they don’t…it’s just a numbering thing. As the Catholic Encyclopedia puts it.
A confusion, however, exists in the numbering, which is due to a difference of opinion concerning the initial precept on Divine worship. The system of numeration found in Catholic Bibles is based on the Hebrew text, was made by St. Augustine (fifth century) in his book of “Questions of Exodus” (“Quæstionum in Heptateuchum libri VII”, Bk. II, Question lxxi), and was adopted by the Council of Trent. It is followed also by the German Lutherans, except those of the school of Bucer. This arrangement makes the First Commandment relate to false worship and to the worship of false gods as to a single subject and a single class of sins to be guarded against–the reference to idols being regarded as mere application of the precept to adore but one God and the prohibition as directed against the particular offense of idolatry alone. According to this manner of reckoning, the injunction forbidding the use of the Lord’s Name in vain comes second in order; and the decimal number is safeguarded by making a division of the final precept on concupiscence–the Ninth pointing to sins of the flesh and the Tenth to desires for unlawful possession of goods. Another division has been adopted by the English and Helvetian Protestant churches on the authority of Philo Judæus, Josephus Origen, and others, whereby two Commandments are made to cover the matter of worship, and thus the numbering of the rest is advanced one higher; and the Tenth embraces both the Ninth and Tenth of the Catholic division.
In other words, for Catholics, the “Worship G-d” and “Don’t have idols” are both 1 commandment, while for most Protestants, they’re 2, while the “Don’t covet your neighbor’s wife or his ass” are 2 commandments for Catholics, but most Protestants combine the two prohibitions.