As stated earlier, it is likely the highest voltage is the "nominal" voltage and the process is fine to operate at that level, meaning the low voltage parts have a lot of voltage headroom to OC with and with them being fast/leaky parts they should behave very well with it.
These are the most power bound to reach their specs. Likely they use the "rarest" of parts that are both relatively fast (so they don't need high voltage) and relatively low leakage. In other chips in the stack these types of parts go to the notebook SKU's but, commonly at these high end chips, no notebook target they can make dual chips boards with such parts.
Yep, it all makes sense now. Thx.