Why would an American think the pledge of allegiance started out saying "under god"?

They're ignorant of historical facts.

Additionally, how many right-wing Christian Americans are aware that "the Pledge" was composed by a National Socialist (or, if you prefer, a Nazi) as a means of encouraging children and other citizens to feel a debt of loyalty to The State?

You are in no way obligated to include that. The pledge itself is reaffirming the fact that if we are legal citizens we will protect our rights. I am pagan so I skip that part of it. But that is up to you.

Because their Christian leaders want their sheep to forget about those things, and tell them this nation is a Christian nation, even though it's false.

I thought that Church and State were not connected in your country [ I am English]

I thought your constitution freed you from this burden.

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Back in England and

....IT IS RAINING

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Because they're ignorant. That's the answer to most questions that start out, "why would Americans...?"

The rules change sometimes.

because, religious people have a habit of ignoring history contrary too their beliefs.

It wasn't.

"Under God" was not in there originally.

ignorance.