With function set, you can select the number of line of your display. 1 ou 2 lines of fonts. By the way, it's also 2 line with a 4 lines display.

About the duty factor, I've read that:
5x7 dots + cursor : 8 common signal (in fact 7+1=8 lines of dots) -> 1/8 duty factor
5x10 dots + cursor : 11 common signal (in fact 10+1=11 lines) -> 1/11 duty factor
2 lines 5x7 dots + cursor : 16 common signal (in fact 2x(7+1)=16 lines) -> 1/16 duty factor

The name of duty is that not all lines of dots are light together; it's a line after the other.
It's matrix display!

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Dufour