It just sounds like 3.5/4 because it was trained on it.
The llama2 is a language model. I imagine the language model behind chatgpt is not much different (perhaps it's better, but not by many months AI research time). It likely also suffers from "mode collapse" issues etc.
But 3.5 also has a lot of systems around it that detects mode collapse and applies some kind of mitigation, forcing the model to give a more reasonable output. Mathematical / logical reasoning questions are likely also detected hand passed on in some form to a separate system.
There's a few public numbers from a handful of foundation models as to performance vs parameter count vs architecture generation. Not being able to compare in detail the architecture of the various closed models nor being more rigorous on training with progressively sized parameter sets, the conclusion at the moment is a general feeling or conjecture.