It’s exactly like that though. Let’s take Narcon Prison as an example:
https://tos.neet.tv/zones/4780
Take a look at the gemstones: Topaz, Opal, Sapphire, Ruby, Mithril Ore and Blessed Shard. As you can see, there’s a 5% chance of any gemstone to drop after killing any monster inside Narcon Prison, but all gemstones have different individual drop rates. Mithril Ore is more likely to drop (27%) and Blessed Shard is the rarest one (0.9%). If you take the sum of all gemstones drop chance, you’ll get 100%, so you can see they belong to the same group.
The same occurs to weapons and armors. You have 1.33% chance of dropping any armor piece. If an armor piece should drop, a second roll decides which piece you’ll get: Primus Rodeleine Shield is less likely to drop, while a blue piece of lv170 armor is the most common roll among armors.
From my experience, the game seems to make 3 rolls. There’s a chance that you can drop one of the monster’s specific loot, one gemstone and one piece of equipment. This happened once when I was farming in one of the HGs and a mob dropped 3 different items.