Hey there, average Terrarian! I thought I should create this post, not because I have seen people who misunderstand, but because I think it's good to know how the drop rates function. I would like to briefly discuss the drop rates of items and how that all "works". Every time you kill a monster, there is a chance that that monster is going to drop an item. Let's take the Magnet Sphere for example. The Magnet Sphere has a 0.25% chance to drop from Blue Armored Bones in the Dungeon after entering Hard Mode. That 0.25% can be simply put as 1 out of 400 chance. You may be thinking, "OK! Now I only have to kill 400 Blue Armored Bones and I'll have it!" This, however, is unfortunately not the case. The 0.25% chance means that on average, it will take an individual 400 kills before the item drops. Each time you kill a Blue Armored Bones the game will randomly generate a random decimal from 0 to 1 and if that number is less than 0.0025, you get the Magnet Sphere.* But the probability goes deeper. It is a common assumption that every time you kill an enemy your chances of getting it on the next kill goes up. This makes complete sense as you know that it takes an average of 400 kills to get the drop, so if I have 1 kill I only have 399 to go before I probably get it. With each kill my chances go up! This, as odd is it may sound (it came as a complete shock to me when I discovered this), is completely false. No matter what happens, or how many Blue Armored Bones you kill, you will ALWAYS have a 1 in 400 chance of getting the drop. Casinos often will use this misconception to trick people into playing games like roulette. When the ball is spun in the roulette wheel, it can land on either black or red (along with some numbers and such, but for now that's not important). What the casinos will do is they will display the last couple colors that the ball landed on. They do this because when someone walks by, they see that the last several colors has been, let's say, black. If you saw that the last several colors have all been black, wouldn't you want to play red? Yes! But not after reading this post! The next time that they spin that roulette wheel, the chance of the ball landing on red is just the same as if it landed on black, 50:50. The same applies to the drop rates for monsters. Killing more monsters doesn't increase you chances of getting a particular drop.
Thanks for reading!
*Note: I am not completely sure that that is exactly how the game randomly selects drops, but that is the accepted way of doing it, and if it is not exactly like that, it is indeed very similar.