Why is the Tx Power not always the same as the typical transmit output power on the AP's datasheet?
The product specification, such as the Typical transmit output power, is defined as its maximum conducted (without Antenna gain) RF output power at all conditions.
The actual output power of the AP varies by its channel, channel width selection (20/40/80Mhz) and data-rate (1~54Mbps, MCS0~MCS7), and it dynamically changes.
It is a general concept that on the condition of lower data-rate, the narrower channel width allows the access point to radiate higher output power, which meets the regulatory limit and IEEE802.11 specification in the meantime. Though there are few exceptions that 6Mbps has bigger power than 1Mbps.
In the controller (or AP) web GUI, to provide a general understanding of current RF power, the Tx Power shows the output power of the lowest data-rate with the current channel and channel width setting. The AP output power, in fact, still varies packet by packet depending on its channel width and date rate.
Due to the regulatory limit, the maximum power varies by channel. Typically, channel 6 in the 2.4Ghz band and channel 149 in the 5Ghz band have the highest output power.
For example, the NWA5123-NI using 20 MHz bandwidth in channel 6 at 2.4GHz shows the max output power of 23 dBm for the USA country code.