on Titan RTX and also Turing-family GeForce GPUs such as 2080/2080Ti, P2P is only supported if/when the NVLink bridge is in place (i.e. only over NVLink). For Turing family GeForce GPUs without a NVLink bridge option, P2P is not supported.
What NVIDIA supports is determined by the tool already run. The behavior may vary by GPU, and is not strictly a function of the motherboard. Since I am not providing a matrix that covers every question for every case, the best determinant of features like this is what the tool reports.
NVIDIA may choose to design a product that supports P2P over NVLink but not PCIE. That is what happened with Titan RTX. I won’t be able to give further rationalization or explanation.
In particular, questions like “why is it this way?” I am not able to answer.
For example, “Anyone knows what makes this difference ?” is a question I wouldn’t be able to answer, other than by saying that is the way the product is designed.