I don't know the answer, but I can see it as being something like:
In 1100's europe, there was land that was cleared for cultivation and forest. Most land holdings had areas of wilderness. They cleared it as the population increased. If the landholder could have instantly increased the population, he would have put the new people to work clearing land and planting crops (probably via leasing it to them as a landlord, or some kind of serfdom, but the mechanics of that don't matter here). So, the number of 'jobs' available on a given landholders property might have been 5000, while the population at the time was 400. There were more job openings than there were people.
Later, factories pulled population away from farms and improved farming allowed the land to feed more people than actually lived on it. This reduced the number of farmers needed as a proportion of the population, so if the population were the same as in 1100, the 5000 jobs we had before might have dropped, perhaps to 2000 jobs, or rather, food prices would have dropped so that lots of people no longer saw farming as the best idea. However, the population had increased greatly. My hypothesis here is that the number of jobs that could have been productively filled was still higher than the population.
This process can't continue forever. Sooner or later, the number of jobs simply crosses over the number of people looking for work. In history, I can imagine a leader, seeing lots of able bodied, desperate, and incredibly cheap manpower seeing an opportunity to raise an army and attempting to expand their own wealth by expanding their lands. That was a natural brake on the population line. Today, thankfully, we don't have any mechanisms that drastically reduce the population whenever people get desperate, there isn't wide areas of undeveloped land that can absorb as many people as we have over above the jobs line, and technology continues to increase productivity at a respectable rate.
So that is how I explain why the jobs line can cross over the population line now, and why we may now be in a situation where we might expect to have less jobs than people who want jobs for a long time.
In 1100's europe, there was land that was cleared for cultivation and forest. Most land holdings had areas of wilderness. They cleared it as the population increased. If the landholder could have instantly increased the population, he would have put the new people to work clearing land and planting crops (probably via leasing it to them as a landlord, or some kind of serfdom, but the mechanics of that don't matter here). So, the number of 'jobs' available on a given landholders property might have been 5000, while the population at the time was 400. There were more job openings than there were people.
Later, factories pulled population away from farms and improved farming allowed the land to feed more people than actually lived on it. This reduced the number of farmers needed as a proportion of the population, so if the population were the same as in 1100, the 5000 jobs we had before might have dropped, perhaps to 2000 jobs, or rather, food prices would have dropped so that lots of people no longer saw farming as the best idea. However, the population had increased greatly. My hypothesis here is that the number of jobs that could have been productively filled was still higher than the population.
This process can't continue forever. Sooner or later, the number of jobs simply crosses over the number of people looking for work. In history, I can imagine a leader, seeing lots of able bodied, desperate, and incredibly cheap manpower seeing an opportunity to raise an army and attempting to expand their own wealth by expanding their lands. That was a natural brake on the population line. Today, thankfully, we don't have any mechanisms that drastically reduce the population whenever people get desperate, there isn't wide areas of undeveloped land that can absorb as many people as we have over above the jobs line, and technology continues to increase productivity at a respectable rate.
So that is how I explain why the jobs line can cross over the population line now, and why we may now be in a situation where we might expect to have less jobs than people who want jobs for a long time.