Farming for the Frozen Turtle Shell is certainly a fun thing to do.
Now, you might be confused as to why you are getting multiples of items "more rare" than the Frozen Turtle Shell; and this is the reason why: the Ice Sickle technically has a lower drop-rate but drops from virtually all Ice biome modes while the Frozen Turtle Shell's rarity is only compounded by the fact that it drops from scarcely spawning Ice Tortoises.
Now here's a similar parallel: you probably get more Hallowed Keys than the Rod of Discord when farming in the underground Hallowed despite the fact that Hallowed Keys have a "lower drop chance." Hallowed Keys can drop from any mob in the Underground Hallowed while Rod of Discord only drops from a single uncommon enemy.
If you're farming, first get some LUCK! Luck potions are not fun to get (requiring pearls from the Oasis) but you could place Ice Torches and Garden Gnomes to obtain a permanent Luck bonus in your farm. Also, search-up a farming guide in order to make the most optimal farm, and make sure to reduce enemy spawn areas to a single accessible place where you can kill them. Only a certain amount of mobs can be on the screen at a time, and this counts mobs stuck in small caves around your farm. Hopefully you can get the Frozen Turtle Shell after making your farm more efficient.