Auto-levels works best when you have a "well-populated" histogram to begin with. The ideal shape of the histogram looks like a mountain peak, sloping off evenly on either side. Sometimes you have a nicely shaped histogram but there is space on either side. What auto levels does essentially is move the sliders on either side to compress the image information and remove unneed data, ie. deepen the shadows and lighten the highlights. I'm sure someone else has a more technical answer and can explain it much better.
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