Maybe it is from watching the pros. I myself average about 210 yards with a one-iron, 185 yards with a three-iron, 165 yards with a five-iron, 140 yards with a seven-iron, 110 yards with a nine-iron—and I don't try to hit the ball at full strength with my irons. Perhaps the average golfer, after watching a pro drop a five-iron shot right next to the pin from 165 yards out, thinks that he is supposed to do it, too. He isn't.
Since so many players underclub themselves on all their approach shots, I am almost tempted to say that you can take strokes off your game simply by using one club longer than you think you should use—in other words, by reaching for a three-iron every time you think you should use a four-iron, and for the seven-iron every time you should use an eight-iron. Try it some day, even though it is too simple an answer to the problem.
What you really should do if you want to improve your approaches is make up your own table of distances. And you can only do this through practice and observation. Keep a record of your iron shots. How far do you really hit a three-iron? How much distance do you really average with a seven-iron? Find out, and then abandon any false pride.
If you keep tab on the distance you can reasonably expect to get out of your clubs, you will soon discover something else that every pro knows. The distance never stays quite the same from day to day. When you are feeling strong and everything is going right for you, the distance goes up. When you are not quite feeling your best, the distance goes down. So, in addition to the obvious factors of wind and damp air, think of how you feel before selecting a club.
Finally, all the club-selection sense in the world will not do you any good if you do not have the right club to select, and there is one club I feel you must carry, the pitching wedge. I suppose that the great majority of amateurs don't even own one, but it is in many ways the greatest golf club ever invented, and you need it to play your best. Almost every pro in the business uses the pitching wedge on virtually every shot from about 100 yards on in. A full swing with the wedge will send the ball high in the air for 100 yards and stop it dead where it lands on the green. By shortening your backswing you can get great accuracy from 80 yards, or 50 yards, or any lesser distance. If you open the face of the wedge, you can hit the ball almost straight up. You can hit a pitch-and-run shot with the wedge. If you close the face and hit down sharply, you can send out a little liner that looks as if it would roll forever but has so much backspin that it bites right into the green and stops.
It is not an easy club to use. You will have to practice with it a great deal. You may find that it is particularly difficult to use from a tight lie, because you have to stroke the ball very accurately with the club face. Any tendency to look up can be fatal. But once you have mastered the wedge, it is great. Most pros use it in even the tightest lies, out of the thinnest grass. It was a wedge shot that I sank from a bare and nasty lie on the 16th hole at Augusta in 1962 for the birdie that I needed in order to go on and win. No pro would think of playing without a pitching wedge. You shouldn't either.