Physically-Based Rendering (PBR) is really a modern tag people use to refer to the trend of making the rendering system "correct" with regard to the real-world physics of light interacting with surfaces, rather than an exact definition of a full rendering system. In other words, I would say PBR is more a definition of the aspiration of the system than what it strictly does.
See here for a good summary of the topic.
Therefore no two systems are guaranteed to try to get to the goal in the same way.
Different systems may model light more or less accurately in the simulation for example - i.e. one might have better global illumination system or better shadow mapping etc. So while the system might model light-surface interaction more accurately, and thus be described as a PBR system, the incident lighting on the surface in the first place might be a bit different giving different results in each system.
So, if given a single flat surface in a scene with one light it's reasonable to expect two PBR systems to give basically the same results. But when it comes to rendering a whole scene with lots going on, there are many variables in play that will probably lead to more obvious variations in the resultant image.