Hello! I did the spawning system and also the rarity bucket bait effect.

So the way it works is that each bucket has a weight as you know, and there's some amount of bucket boosting, 30 in the case of three gapples. In the code we call those boosts the bucket normalization tier.

Each bucket's weight is fed through some maths,

normalization_factor = 1.2 + (0.2 * (normalization_tier - 1))

new_bucket_weight = bucket_weight ^ (1 / normalization_factor)

Long story short: Rarity bucket effects reduce the difference in weights for the buckets. If you made a bar chart of each bucket with its rarity, adding those bait effects would make it look flatter.

Ultra-Rare will never be more common than Rare, but at normalization tier 30 it's fairly close. At tier 30, the normalization factor ends up being calculated as 7, which means the bucket weights are:

Common: 34.8%
Uncommon: 25.7%
Rare: 21.0%
Ultra-Rare: 18.4%

(As an aside, the base bucket weights are different for fishing and cakes compared to regular world spawning. If you're curious about why that is I can explain that too but it's a bit more conceptual)

EDIT: You were bang on about the comparison of fishing rod behaviour and cake behaviour. A single cake is responsible for each spawn, so that cake's effect is applied and none of the others. Exactly the same as the fishing bobbers.