My expectation is that two worlds with the same 'world seed' would have the exact same terrain generation, but this is not actually the case.

As an example generate a few worlds with seed '1' in creative mode, and fly to the edge of the jungle just northeast spawn, around coords 270/90/225. While most generation is the same, there is variation in jungle tree size and ground clutter, e.g. some worlds may have melons on the ground there, while others do not, despite having the same world seed.

I have only witnessed differences in Jungle and Mega-Taiga biomes, which makes me speculate that tree generation (and tree trunk size of 1 or 2) happens outside of the 'seed RNG', and then this interacts with the population of ground clutter (melons/pumpkins/mushrooms/deadbush/etc).

It is not a huge deal, but sometimes speedruns and various challenges use the same seed to have players in different single-player worlds on equal footing, and when there is variation like this, it can damage that experience (e.g. one player gets melons for health pots and the other does not). Given how much of the terrain generator already does correctly utilize the 'seed RNG', it seems like this might be a very easy bug to fix.