The difference between "possibly cancerous" and "fully cancerous" is that the former is not confirmed to have the property of causing cancer.

Radiation on the other hand is known to be carcinogenic.

To use your analogy, we know that there are bacteria that cause infections and bacteria that are harmless to humans. Let's say we have bacteria A that is known to cause infection but not always in everyone. Then we have a bacteria B, which is potentially able to cause infection. We don't know for certain that it can, but we also don't know that it can't.

And yes, it's a pretty fucking useless designation, and WHO is wasting everyone's time and causing undue panic. Let's not forget how they completely fucked the world with their atrocious handing of Covid in the early stages of the outbreak.