The HD44780 does not "know" how many lines are on the display! If you move the cursor to character cell 20, that's the first character cell of the second row. Character 40: first cell of third row; character 60: first cell of fourth row. The HD44780 only has 80 character cells, no matter what size LCD is connected.
Now, I don't know if the code in the Arduino library does anything cleverer than that and re-maps the display at all!