The term “round dot” refers to the screening algorithm that tends to draw round shaped dots.
Let’s not forget, however, that the original digital file is made of square pixels. These pixels are the binary information that is sent to the laser to write the image: write / don’t write = on / off.
Therefore, a round dot will be formed by a whole number of pixels trying to create a rounded shape. But they are many little squares grouped together.
A tonal value of 50% at 133 lpi (about 54 l/cm), at 2400 dpi, is made up of 162 pixels that create a more or less round shape.
A 2% of the same screen is made up of groups of 6 and 7 pixels
It will be therefore more difficult to create a rounded shape with 6 or 7 little squares.