Why is it important to ensure that the implementations are comparable? Why would someone contribute one that isn't? And if a language has to do something unus

Why is it important to ensure that the implementations are comparable? Why would someone contribute one that isn't? And if a language has to do something unusual because of specific quirks, I'd argue that's interesting by itself, and demonstrates how languages are similar and different (the stated goal of Rosetta Code). Rosetta Code is specifically not a collection of library code. You're not supposed to take code from there, so it doesn't matter whether they all implement exactly the same thing. The advantage over GitHub is that it's much easier to find what you're looking for, and the code you will find will (supposedly) be written explicitly for expository purposes.