10 dBm limit - why?

The international versions of Series II and IIb modules have a power limit of +10 dBm (exactly 10 mW).

So far, I took this information for good and did not investigate further, but now I need to better understand the reason of this limit.

I read that ETSI defines a limit of 100 mW EIRP, or alternatively 10 mW/MHz of bandwidth, for the ISM 2.4 GHz.

Now, Zigbee allows 16 channels with 5 MHz bandwidth, and this should limit power to 80 mW EIRP = circa 19 dBm with a gainless, isotropic antenna.

So I don’t understand the 10 dBm limit. Of course I am missing something or have wrong information. Can some expert please help me?