Makes sense if you think about it. Its figured from 1900. The ages and years are proportional. So in 2000 someone born in 1901 would be 99 this would add to 100 same for someone born in 1999 this person would be one in 2000 thus added together would also be 100. If someone was born in the 1800's their number would be 211, the 1700's would be 311 and so on.
It was neat though.