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The distortion appears in the top and bottom edges of the trapezoid. With no distortion the edges would be straight lines like this: http://www.part15.us/files.p15/tn_AM25%20Trap.jpg
The last pattern picture you posted looks a lot better but it still shows some distortion.
What is happening with the trapezoidal pattern is a plot of the audio input vs. the audio output with the output in the form of the RF envelope. Think of plotting the input vs. output of an audio amplifier. You would see a straight line tilted upward or downward. Here, since the envelope has both positive and negative going excursions with the audio we get both negative and positive sloping straight lines (if not distorted) with the audio.
I will try to find a link for you on this. My older ARRL Handbook has it but I guess they dropped it since mine was published.