This is pure speculation, but I wonder if the likelihood of hallucination has something to do with the amount of "generic" tokens the model emitted before - I.e

This is pure speculation, but I wonder if the likelihood of hallucination has something to do with the amount of "generic" tokens the model emitted before - I.e. tokens that themselves don't depend on the answer, but still restrict how a grammatically correct sentence would have to continue. E.g. in the OP's example, the first tokens the model emitted were "The species Brachiosaurus brancai was reassigned to its own genus". This is just a reformulation of the question and doesn't itself contain any new information - but it does make it a lot harder for the model to make a turn into a "I don't know" direction, because this would lead to a grammatically incorrect or at at least very awkward sentence. In contrast, in your question, the model immediately started with "Unfortunately", which essentially set it on an "I don't know" trajectory right away.