I think the best way to explain the problem would be a modified version of the desert island mashal. Lets start with 100 boys and 100 girls shipwrecked on a desert island. The first year 80 boys and 80 girls get married. The next summer shipwreck season another 100 boys and 100 girls arrive on the island. the 20 first year single boys compete with the 100 new boys over the 100 new girls, but no one is interested in the 20 “old rejected” girls from the first year. The second year also has 80 chasunas: 70 new boys and 10 of the old boys marry 80 of the new girls. At the end of the second year we have 10 single first year boys and 30 single second year boys, but 20 new and 20 old single girls. The third summer another 100 boys and 100 girls are shipwrecked and the pattern continues. Can you easily see how we quickly have many more older single girls then boys?
I think this is a much more accurate depiction of the situation.
PM: Yes, and I think yours is a rather accurate description of the reality.
But it is important to notice (in your example) there is always an even number of unmarried men and unmarried women.