The following is copied directly from the IARC Q & A on their monographs: 
"What does the classification mean in terms of risk? The classification indicates the strength of the evidence that a substance or agent causes cancer. The Monographs Programme seeks to identify cancer hazards, meaning the potential for the exposure to cause cancer. However, it does not indicate the level of risk associated with exposure. The cancer risk associated with substances or agents assigned the same classification may be very different, depending on factors such as the type and extent of exposure and the strength of the effect of the agent."
IARC is limited to assessing hazard only by their own rules. Which is the basis for much of the criticism of IARC.