Very close to this area (about 5 meters from the AP, other side of the till area), I saw a surprisingly weak signal on 5GHz (~-65 - -68dBm if I recall)
The power output is reduced from 20dBm to 14dBm because routerOS is told that the antenna is 6dBi gain.
I suspect the antenna might not be very good or not really be 6dBi :-/ Who knows.
cAP ac XL will reduce it's TX power due to regulations in Europe. (ETSI Max power is lower than eg US where you could hit the radio limit).
(You are not limited to the indoor freq list if used indoors. Use installation "any". The other freq allow for a higher power. There are two sets of freq: "indoors only" and "also outdoors".)
IF regulatory domain max EIRP is dictating the max TX power
of the radio, then any high gain or directional antenna (higher gain antenna is always directional to be able to make that gain) will emit LESS total power than an ideal omnidirectional antenna. (The total power emitted is the TX power of the radio). This means you will have directions in which the power is equal
to the ideal omnidirectional, and directions in which it is lower
So somebody measuring the received signal strength indoors at the client, will likely find that the cAP ac XL is weaker than the cAP ac. (The signal path comes from many directions due to reflections)
Concluding that therefor the cAP ac XL is inferior to the cAP ac is wifi being contra-intuitive.
The cAP ac XL antenna gain amplifiies the received signal from the client! That is the usual benefit of an AP with higher antenna gain: "amplifying the reception", and as such extending the range.
This is mostly an improvement as the clients are weak transmitters.
For a long range (eg PtP or PtMP) , both sides should have high gain antenna .(and best similar gain, for symmetric transmission needs).