My apologies if this has already been answered but how does a Catholic properly explain to a non-Catholic why our version of the Our Father is different than the way Protestants recite it. "We" do say "For thine is the kingdom, the power, the glory now and forever. Amen". during Mass but it's after the Priest says his special prayer. Why the difference? When his disciples asked Jesus how to pray, our Lord didn't include the prayer that the Priest says during Mass. So, how do we explain the reason the Church does that, to non-Catholics? Is there anything in the Catechism on this? Thanks!

The additional words aren't part of the prayer. They were written in the margins of a Bible (I'll have to research which one), and later added in as part of the prayer.