I assume that leek moth and carrot root fly are too stupid to work their way under net on the ground, and try to fly into the net, that being the source of the delicious smell. Things that normally walk on the soil surface such as flea beetles will I assume simply walk under the net, since the soil is naturally rough, unless you weigh all of the edge down.
I have a huge problem with pests on chinese cabbage,, and do use fine netting.