They probably just average the cost out to keep things simple.
I mainly use 3 straight chisels for most of my inletting. One is about 3/16" wide, maybe a tiny bit less, and the other about 1/2". Then I use one about 3/32" wide for underlug, thimble, key and trigger slots, etc.
I used to buy all sorts of chisels, antique ones etc, some nice Pfiel ones, but I like my homemade ones made from Swiss files the best. I have some big gouges I never do use anymore, preferring the drawknife, spokeshave and planes for removal of a lot of wood.
I'll buy parting tools though I have made them also.