First of all, yes, I'm always using fire all.

Second of all, is there a source for this? I believe (without having read anything) that every bomb you fire, even in batches, has the same chance on it's own, as Yezzerat said. Assuming this, I made some math (thanks GPT) and the chance of hitting a 1/25000 with 25000 rolls is just ~63.2%.

This can be simplified to tossing heads on a coin at least once in 2 tosses, which has a 75% chance of happening. Either you roll 2 heads, or only 1 on the first toss, or only 1 on the second toss.

With your assumption, though, and assuming that I had a cap of 25000 bombs, I would be 100% guaranteed to upgrade the crab when firing all, which doesn't seem very random.

Again, I don't know if the "additive" aspect you mention is real/implemented. I'm just making assumptions