Spigot CraftEntityEquipment deliberately decreases drop chances by 0.1 on set and adds 0.1 on get.

    private void setDropChance(EnumItemSlot slot, float chance) {
        if (slot == EnumItemSlot.MAINHAND || slot == EnumItemSlot.OFFHAND) {
            ((EntityInsentient) entity.getHandle()).dropChanceHand[slot.b()] = chance - 0.1F;
        } else {
            ((EntityInsentient) entity.getHandle()).dropChanceArmor[slot.b()] = chance - 0.1F;
        }
    }

    private float getDropChance(EnumItemSlot slot) {
        if (slot == EnumItemSlot.MAINHAND || slot == EnumItemSlot.OFFHAND) {
            return ((EntityInsentient) entity.getHandle()).dropChanceHand[slot.b()] + 0.1F;
        } else {
            return ((EntityInsentient) entity.getHandle()).dropChanceArmor[slot.b()] + 0.1F;
        }
    }

You can use the provided test plugin (https://github.com/totemo/TestDropChances) to spawn mobs with known armour and hand drop chances.

If you spawn mobs with `/testdropchances 0.099` note that they will not drop their equipment on death ever when killed with a non-looting sword.

The NBT fields of the mob are incorrect by the 0.1 offset, as can be verified by vanilla commands:

/data get entity @e[type=minecraft:wither_skeleton,distance=..20,limit=1] ArmorDropChances
/data get entity @e[type=minecraft:wither_skeleton,distance=..20,limit=1] HandDropChances

When the mob dies, these 0.1 offsets are not taken into account by CraftBukkit n.m.s.EntityInsentient.dropEquipment() and Spigot-Server n.m.s.EntityInsentient.dropDeathLoot().

The relevant drop chance arrays are initialised to expected vanilla drop chances:

        Arrays.fill(this.dropChanceArmor, 0.085F);
        Arrays.fill(this.dropChanceHand, 0.085F);

This is the situation in the current build but also applied about 5 or 6 months ago in Spigot 1.13.2. I didn't follow up on it at the time thinking it was a temporary situation.

The offsets in CraftEntityEquipment are clearly deliberate, but I would liken them to junk DNA. Taking them out would probably resolve this.