It‘s not about whether the client devices can hear your access point, it’s about whether or not your access point can hear the client. Transmit power has no impact on whether or not the receiver antennae in the XT12 can hear your iPhone or iPad when they’re far away.
I understand what you are saying, however, you do have 2 people with similar kit both saying they have no issues with their wifi networks and are actually very happy with the performance.
The transmit power of the clients does come into play here, lets not forget that, and the advances made in mobile device comms tech has come on leaps and bounds.
How do you carry out the tests, would I be correct to say that you follow a very specific method?
Do the conditions of your test reflect real world conditions?