I think this one's been posted somewhere...but it bears repeating. El Comandante may delete at a later time if he deems it best to do so.
Inertia! Just no way to get around Newton's Laws regarding motion.
Remember the old trick of jerking a tablecloth out from under a table setting? This is the principle at work here.
We have a round in the chamber and one round in the magazine.
Bang! The slide starts to move as the pistol torques up and back. The slide holds the last round slightly below the feeding position until it moves far enough to uncover it. Just as the magazine spring is struggling to move the round into position, the round is in a sort of "Limbo" while the pistol continues to move backward in recoil. The round obeys Newton, and stands still while the gun is moving away from it.
The magazine spring catches up, and gets the round up and into the underside of the feed lips, but because the pistol pulled backward away from it (Even though the pressure from the slide drags the round backward in the magazine)...it settles down forward of the feeding position. At this point, if the magazine spring is strong enough to keep it there, the slide pushes it ahead of the extractor. The pistol either fails to go to battery with the round fully chambered, and the front of the extractor rammed against the back of the rim. Extractor breakage is an eventuality.
If the spring isn't strong enough, the round is forward of optimum feeding position just as the slide smacks the impact surface in the frame, and triggers a second recoil impulse. The gun makes a short, hard jerk upward and backward...and the round is in limbo once more because the mass of the round has caused the magazine spring to compress slightly. The round...already too far forward in the magazine...jumps the follower, and is free of the magazine. The follower pushes the slidestop up as the slide moves forward, and the slide locks. If the magazine spring is weak enough, the next to last round will be ejected from the magazine, and the last round feeds. Ever found live ammo among your brass? Heeere's yer sign!
The problem is two-fold. One is the spring that has fewer coils to make room for the extra round. There is ample tension to feed until it gets to the last round...and tension is at a mimimum...but sometimes it can happen before the last round. Upping the spring rate helps, but doesn't address the other issue.
The other part of the problem is the smooth follower. Browning knew how
inertia would affect things, and he put a small dimple on the top of the magazine follower. The dimple's function is two-fold. It adds a small amount of height to the follower in order to give it a "Leg Up"...and it stops the forward movement of the round. More accurately, it keeps the pistol from moving out from under the round in recoil. In this function, it's basically a back-up for the spring as it fatigues, and provides a better opportunity for the round to stay in position to be stripped from the magazine by the slide instead of being pushed ahead of it...or... in the extreme cases, escaping from the magazine completely.
John Moses designed a 7-round magazine and he put a dimple on the follower for very good reasons. Whenever we try to change things in order to "improve" the gun...we very often cause problems. There just ain't no such thing as a free lunch, I'm 'fraid.