Pokemon Go has made a couple of tweaks to its egg pools.
"Egg pools" are how Pokemon Go determines what Pokemon hatch from eggs gathered at PokeStops. Egg pools are technically divided into two groups - by egg type and by rarity. For instance, a Poliwag is much more likely to hatch from a 5 KM egg than a Chikorita, even though both pop out of the same type of egg.
In addition, Pokemon Go distributes eggs based on the general rarity of the Pokemon inside rather than by what type of egg it hatches from.
Back in October, Pokemon Go added three more basic Pokemon to the egg pool after adding them to the game. Shuppet and Bandette were added to the 5 KM egg pool while Sableye was added to the 10 KM egg pool.
The Silph Road, which discovered how eggs worked several months ago, has determined that all three Ghost-Type Pokemon are "rare" eggs, meaning that players will only have about a 0.8% chance of hatching them from eggs.
In addition, the Silph Road also determined that Shuckle is actually a "super-rare" hatch that will appear in 0.4% of all eggs. The group had previously identified Shuckle as an "ultra-rare" that only appeared in 0.2% of all eggs.
None of these are groundbreaking changes or discoveries, but this new information helps provide additional insight as to how Pokemon Go functions.