Have you ever known people that are quite genius? They amaze you with the intelligence they have and yet in everyday life they just do not have the brainpower that god gave a goose.
I married two of these type of people and I always had to wonder how things like the universe, God, science and quantum physics can be spoken with ease...and yet the simple things absolutely escape them.
Actually, it's a myth that this a smart-person syndrome. It's more descriptive of a number of potential undiagnosed problems: ADD or ADHD, sleep disorder (delayed sleep phase syndrome, e.g.,--sleep deficit in general), problems with blood sugar regulation, a mood disorder (possibly mild), OCD, schizotypal disorder, an autistic spectrum disorder.... In other words, regardless of intelligence, people can have undiagnosed issues that affect everyday living and decision-making and follow through in a way that is maddening to the people around them (and even themselves, if they recognize it). It can be anywhere from mild to severe.
And the more intelligent a person is, the later they are likely to be diagnosed--because they have been successful in employing creative coping skills up to now. [And the more frustrating it might be for loved ones who don't understand this, because they SEEM like they should be able to handle everyday, ordinary things...they certainly are intelligent enough.
Personally, I'm one of those--late life diagnosis of ADHD, primarily inattentive type, which we are just now realizing may really be due to an underlying chronic (and very difficult to treat) sleep disorder (delayed sleep phase syndrome). I can take statistics and trigonometry and understand all of the concepts--I know what I'm supposed to be doing--but I can't balance my checkbook to save my life. I am a master at planning, coordinating, and organizing, but when my sleep or blood sugar is off-kilter, I can leave one appt. and forget where I'm supposed to go next...leave the room and forget what I was going after...etc., etc. It's a constant balancing act to stay on top of that--the sleep/ blood sugar AND the spaciness/ forgetfulness.
I wasn't kidding on a different thread when I said a friend of mine nicknamed me something to do with my forgetfulness, but I can't remember what it is....
Plus, I have a highly intelligent friend with bipoar disorder (tends to mania end, seems unusually 'uplifted' most of the time) who is so distractable and disorganized that I had to place a boundary on our friendship that says I will visit with him outside of his [mountain of a mess!!!] home and NOT try to go shopping with him again. I'm distractable enough on my own...I have a hard enough time keeping myself on track--I can't keep track of his tangents on top of my own! And yet, we can have the deepest discussions on a very abstract level.
So...whomever it is you're noticing this about, watch and see if you notice any of these other things too. And just remember...dumb people can be ADD or bipolar too....