You will have to ask lots of other protestants the same question.
The Lutheran (Protestant) and Catholic division of the commandments both follow the one established by St. Augustine, following the then current synagogue scribal division.
source: http://en.wikipedia.org/wiki/Ten_Commandments#Catholic_and_Lutheran_Christianity
Colossians 2:8 See to it that no one takes you captive through hollow and deceptive philosophy, which depends on human tradition and the elemental spiritual forces of this world rather than on Christ.
Mark 7:13 Thus you nullify the word of God by your tradition that you have handed down. And you do many things like that.”
My friend and fellow Christian. I am not here to put you down or argue about your faith. I have pondered and prayed about these things for years. My daily prayer is that I would not be deceived. I do not put faith in any man. I have put my faith in Christ alone. The churches, over the centuries, until today do not resemble, in any way shape or form, the original first century church that Christ sanctioned. Jesus presented himself as a Jew and he still is. He was not a Catholic nor a Protestant. He is sitting at the right hand of the Father and will return to receive those who love Him, follow His commandments and know Him and walk humbly with Him. The forces of deception are everywhere and wide is the path... personally I take my refuge in Him alone.