My understanding of Group 2B is that it’s assigned to whatever IARC researchers get enough lobby money for to classify in some way. They’ll give some lab rats a gargantuan dose of the substance, something will happen, and 2B can be assigned.
Put another way, they found all the Group 1 definite carcinogens by the early 2000s and needed a reason to keep researching after that. Of course there are some new substances that pop up regularly that need to be investigated but beyond that there’s a significant established base of knowledge.
With the RF specifically its interesting that some groups have correlated it to negative health outcomes, but nobody as of yet has shown how it can even affect the human body, let alone cause cancer.