I will say exactly same as my friend and 'senpai' (Nakamazi Itchigo) :heh: tells me.
You're like me, we mark the under line way to hard, we create this false 3rd shading tone. There are only two kind of shading. Hard Edge and Soft Edge. Soft Edge when something is curving and Hard Edge when something is completely block from reflection (example : Being under a roof when the sun is directly above the roof (standing under a cube when reflection is above the cube) ).
When we go for realistic styles we always haft to do shadings, these are what make drawings look more realistic. So eyes are round so they have tons of shadings. So the inner eye (white part) needs more shadings to tell us that it's not a square, and the under line needs tons of shadings as well since it's not a hard edge, same with top but mostly you darken it much more since you have those eye thingy there (can't remember name but hopefully you understand.)
Hope this help
If not then I am sure that Nakamazi Itchigo will gladly tell you more than I did *_*