You shouldn't be surprised by the confusion. I sometimes suspect it is intentional on the part of restaurants; "ahi" sounds a lot classier than "tuna", because it doesn't make you think of canned tuna.
I understand your point. But when someone starts out their sentence with "I wish I knew what to tell you" you would like to think that they knew that Ahi, AKA yellowfin or bigeye tuna, was two species of tuna.
