Archer C7 V5 2.4GHz transmit power is limited to 17dBm but set to 20dBm

Actually, my question is in the header.
I have Archer C7 V5 with 21.02.1.
2.4 GHz transmit power is set to 20 dBm (the maximum for my region), but iwinfo and LuCI show that the current transmit power is 17 dBm.
I also tried changing my region to the US and setting Tx power to 24 dBm, then iwinfo and LuCI showed 20 dBm.
Why is that happening and what is the real power? Should I set the US and 24 dBm to get the REAL 20 dBm?

This issue is related to 2.4 GHz only, 5 GHz works just like set up.