Hi, WiFi signal degrades with distance from the AP, -65dbm has become the industry standard as the recommended limit of signal strength/coverage to ensure that the connection from the device to the AP is still stable, once the signal drops below -65dbm the connection starts becoming unstable, i.e. packet drops, intermittent loss of connection, etc. You will still be able to get a connection at -72dbm but you would see increased latency and potentially packet loss dependent of how much the signal strength fluctuates
Right, I guess the more cynical answer is that 65dBm has become the industry unspoken standard for the minimum RSSI that a half decent wifi device should be able to hold a connection at. If you fall below there, then you're likely to experience more and more clients that can't hold a signal.
Wifi client antenna / RF design is pretty inconsistent, just like how you have good AP models and crap AP models, the same holds true on the client side. I've had some awesome laptops that can hold a connection down close to -80dBm and pick up wifi where none of my other devices can see wifi, but I've also had the opposite built into gaming consoles where -65dBm was already not enough for Netflix streaming.
I also consider SNR in this scenario. at -65dBm "in a normal operating environment" it is safe to assume that the signal strength is strong enough to not be effected by WIFI & none-WIFI RF noise. Obviously there are instances when the noise floor is much higher and you can still experience issues at -65dBm