Driver default Maximum transmit power

I had assumed the Maximum transmit power is set per (each country's) regulatory domain. But then I noticed for the exact same channel, the "driver default" MTP is different for two of my openwrt devices with different hardware (6dBm difference in fact).

Where does openwrt get the different "driver default" Maximum transmit power for different hardware? Is it determined by the chipset maker (atheros/mediatek/broadcom/etc.)?

Do you live in EU?