Here my view on the subject
even if the radio specs say it can transmit at 22dBm, it will (almost) never do so
1) this power is measured at the transmitter output, not at the antenna
2) if you configure the power level to max, the access point will still throttle it down to the max ERP/EIRP for the regulatory domain
3) be aware the access point has multiple radio's and when using beamforming,
the combined EIRP of all the radio's must still comply to the max EIRP of the regulatory domain
4) internal and external antenna (even omnidirectional) have a specified "gain"
this gain of this antenna must also be taken into account when calculating the EIRP
5) an access point with internal antennae will have the same electronics as the external model.
if the antenna gain is 2dBm and the max EIRP is 20dBm then the transmitter power may not be higher than 18dBm
if beamforming of 2 radio's/antennae adds another 2dBm ,then each transmitter may not transmit more then 16dBm
there will be some more rules why the transmitter reduces its power resulting in the 12dBm of your screenshot
the extra power is "reserve" when you use an external antenna PLUS much loss in the cable between access point and antenna
then the transmitter is able compensate for the loss in the cable
-> the access point with internal antennae "knows" it's antenna gain and cable loss and will never transmit at max power
