You have excellent questions!
Several things happened
1. Many people were not home owners, so an increase of ownership of say 40% to 50% would be 10+ million
2. Money was flowing freely, due to many factors
3. The population being born does not affect housing purchases directly
What website are you using? I think Census data is helpful, but there have to be millions of people that are not counted. If I was from Honduras and hung out at Home Depot for a job, I wouldn't want to declare myself publicly.
Check out this interesting story on NPR. It's the best description I have heard about the housing market over the last 5 years.