that is just what the order 2 and 3 coefficients are capable of. order 2 (biquad) clean to -72 dB is pretty good. order 3 to -108 db very good. i mean, i cannot hear that quietly at that frequency. can you?
choosing what settings you want is all about the task at hand.
The "big knee" you see is simply due to the resolution of the display. When the plot~ object is doing a logarithmic display the first points are very far apart while the later points are very compressed together.