Raspberry Pi GPIO TX Power

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Raspberry Pi GPIO TX Power

Hello all,
Having had success with using the GPIO pin on a Raspberry Pi for WSPR and getting 6000km spots on a piece of wet string, it got me wondering about the TX power.
All the How-to guides state that the TX power is 10dBm (10mW), how was this calculated?
Simple ohm's law and I2R work out around 200mW based on 3.3V and 50R.

Can anyone explain why it is 10mW and not 200?