Under inorganics
the "leather-hard" are materials used in pottery. They are not actual leather.
the "leather" entry is also not really leather and is a stone used in Kobold Camp.
Under creatures
you are seeing exactly what is expected.
As you may be aware, creatures have various tokens of USE_MATERIAL_TEMPLATE. Going into a creature, such as Leatherwing bat, you'll see skin, muscle, bone, etc. Each of those lines come from a USE_MATERIAL_TEMPLATE:SKIN:<material template>. The new material identifier (SKIN) would be changed to BONE, MUSCLE, and so on and so forth. These materials can also be coming from a BODY_DETAIL_PLAN that the creature is using.
However, when a creature is only made up of one material, there is no further need to go into the creature to specify the material. That material is brought up to the top and has the creature name prepended to it to describe that material. So in creature_masterwork.txt, there is a creature [CREATURE:ANIMAL_TOUGHLEATHER] with [NAME:tough:tough:tough]. If you filter the creature list to "tough", you will see "tough leather" because there is no other material in that creature. In this case, the leather material is LEATHER_EXOTIC which has a name of "leather". (You will also see a creature named "tough" which has all of the other materials for BONE, MUSCLE, etc.)
For this reason, all of the leathers, feathers, scaleplates, chitinplates, etc. get their own CREATURE entry so that they are no longer hidden under CREATURE:ANIMAL (blank name), CREATURE:ANIMAL_RARE ("rare" name), or CREATURE:ANIMAL_EXOTIC ("tough" name). I then remove those materials from the creatures mentioned.
And as I've mentioned previously, I didn't fix Suede. The [CREATURE:SUEDE] has two materials defined in it and is a blank entry about 30 pages down the list, by the golem creature.