Here is an example.
Diver Joe tries static apnea. After some practice, he can overcome the urge to breathe, and in the pool, with his friends watching, he blacks out at 5'30".
He keeps practicing, (minus the blackout).
One month later, he has his friends watching in the pool again. This time he blacks out at 6'55".
So, in this fictional example, we eliminate the effect of overcoming the urge to breathe. In both cases the guy blacked out. So, the 'time to blackout' is actually a much better indicator of his body's use of oxygen. In this situation, the time to blackout increased, so one cannot say that Joe held his breath for longer because he 'overcame the urge to breathe for longer.' Clearly his body either stored more oxygen, or he used up his O2 at a slower rate.
Now, the question is:
- Is the above situation realistic?
The answer is sometimes. It depends on the type of static training. Tom Sietas, world record holder in static, has made it clear that he has achieved near 10 minutes in static, doing little except practicing static. Note that he does no warm ups, goes (and reaches) the max time on the 1st attempt. Using this method his 'time to blackout' has improved for two years or more, but he trains about 5-6 days per week.
Other divers aren't so lucky. Using more traditional training techniques, I know of many divers who would black out in static at 6'20" four years ago, and still black out at 6'20" today, despite all that static training.
My personal belief is that for actual diving, the best way to make more efficient use of oxygen (and to teach the body to store more oxygen) isn't static training, but rather breath-hold exercise, such as apnea hiking, apnea stairmaster, etc... though that type of training is very stressful and can only be done about twice a week, unless special precautions are made.