The best analogy I heard to illustrate this was (again)...alchohol.
Drink a bottle of whisky once per year and you are carried home (incapacitated).
Drink a bottle of whisky every day and you can still walk home from the bar (inebriated, but not incapacitated).
Familiarity with the effects of being inebriated, enable the brain to develop a form of 'work-around' function. An alchoholic develops a capacity to stagger the right route home from the local bar, get the key in the door and fall into their bed at home.
Exit bar, turn left...stumble stumble stumble...turn right...stumble stumble stumble...turn right..stumble stumble stumble...open gate...stumble...key in left pocket...fumble fumble...open door...stagger stagger...up the stairs....stumble...open bedroom door...stagger....fall onto bed....
They could not, however, perform the same task if they were in a strange city...and had to work out their location, route and use a different type of key/lock in a different door.
Exit bar, turn left...stumble stumble stumble...where am I?!?...stumble stumble stumble...turn right..stumble stumble stumble...this isn't my road?!?...stumble stumble stumble...give up.....sleep behind dumpster...
With divers...it is quite possible (and I believe this from personal experience) that the 'ingrained' training and experience can still be performed when under severe narcosis (depth/time awareness, buoyancy, following a strict dive plan). However, anything that deviates from these 'ingrained' or conditioned behaviours/skills and presents itself as novel... requiring swift and accurate planning and decision making...will be byond their ability to cope with.
The danger of this is that it can lead to the perception of competence, when the reality is that you are severely detrimented. But you will never experience the severity of that detriment unless you are presented with a scenario for which you are not ingrained to react to.
The more experienced the diver...the more situations they have been ingrained to react effectively to.... hence the less likelihood that they will be presented with evidence of their inebriation.