What the troll named bgh, and most others, don't understand is that the physical demands of each event change AND SO DOES THE PHYSIOLOGY OF EACH ATHLETE WHO SPECIALIZES IN THEM. Take two runners running 800m. One is a 400m runner, who can tolerate 17mmole lactate without breaking down in form or pace. The other is a 3000m guy, who cannot acquire more than 7 mmole, as he has not done the lactate-tolerance work and is not genetically predisposed.
If both do the first 100m in say, 13.0 sec, the physiological effect is very different. For the 3K guy, he sprinting all out, and will soon have to drop from the race. The 400 guy is comfortable and can continue, with a gradual slowdown, for an overall positive split, say of 26/28/28/30=1:51. The 400 guy wins of course.
If instead the 3k guy goes out in say, 14.0 sec the first 100, then his lactate is tolerable (under 7mmole), and he can continue without decrease of pace, as his superior aerobic engine dissipates any further addition of lactate. So running this way, he can run 28/28/28/28 and contend at the wire for the win. So two athletes, two strategies for the win.
Now the 800 specialist has the lactate tolerance of the 400 guy, and also the aerobic support of the 3K (or at least 1.5K) guy. So his best race will fall in between the two: 27/28/28/29. So you can see where the typical 2-sec. positive split for an elite 800 race comes from. For those who are chosen and train the event, it has a "special" characteristic, due to a higher lactate tolerance than found in distance events. Distance races do not start out near a sprint the first 100m, as it would impose too high a lactate load for the remainder of the race. For the 800 it can be done, due to the special training (and genetics, not all runners can build high lactate)
This effect is even greater in the 400. No top 400 runner will ever win against serious competition with even splits. They would feel as if they are jogging the first 200 and losing the benefit of accumulating lactate to run a faster overall time.