“ ‘You see, but you do not observe. The distinction is clear. For example, you have frequently seen the steps which lead up from the hall to this room.’ ‘Frequently.’ ‘How often?’ ‘Well, some hundreds of times.’ ‘Then how many are there?’ ‘How many! I don’t know.’ ‘Quite so. You have not observed. And yet you have seen. That is just my point. Now, I know that there are seventeen steps, because I have both seen and ob…
While good fiction, this is pretty dumb. Did Sherlock count the number of tiles on the wall as well? What about the colour of the carpet or the number of cracks in the ceiling? Knowing how many steps there are is useless information and wasteful to retain.
The implication is that Holmes perhaps had a brain that was particularly well suited to such trivia, and a habit of observations. With practice comes skill in making and retaining such observations.
Now, the stair example is indeed a bit much. I doubt holmes would have counted the steps in every staircase he saw (though then again, if one practiced making such observations....). But Holmes did enjoy throwing people off guard, so knowing the number of steps in ones’ own home might indeed be a useful tidbit, precisely since it would offer the opportunity to throw Watson off guard.