PUZZLE #2 - 3 Witnesses

We're not even into the body of the record yet. It's the last line of the "TESTIMONY OF THE THREE WITNESSES" that give us our second PUZZLE. It says;

"And the honor be to the Father, and to the Son,
and to the Holy Ghost, which is one God. Amen."

Father, Son and Holy Ghost are one God? I believe that to be true, but Mormons don't. Neither do they teach this concept, commonly referred to as the Trinity.
Why does it say so here? This is not Mormonism, but it was the concept believed by all of the men associated with the Book of Mormon in 1830.

Show this to any Mormon and you will get only one explanation. They will say something like; 'It means that Father, Son and Holy Ghost are all one in purpose."

Absurd. Perhaps they are thinking that it says that the three are "one". They will need to read it again. It says that the three "are one God", not that the three "are one."

Words have meaning so that we can use them to express ideas. "One God" means "one God", not "one in purpose." Here then is our second PUZZLE from the Book of Mormon: Why does the Book of Mormon teach something contrary to what Mormons believe and teach?