Terraria totally has some AI deciding drop chances right?

Once I get to Hardmode and am relying on an enemy dropping what weapon I need, it refuses to show up. I could be hunting Crimson Mimics for three hours and end up with nothing but dart pistols, hooks, and that fingernail thing. So I'd just give up and go through without Life Drain, it's fine. Then the very next world, I'm going as Ranger, kill a Crimson Mimic while farming souls and oh look, there's the Life Drain! Control+F+R any weapon with its dropper and it will be the same story each time. This game has a gremlin determining the exact item you want and erasing the drop chance to maximize your playtime.