It is obviously down to who has sent in samples. I know for example that my uncles grandson has sent a sample because I recognised his name from the matches. Also, knowing the calibre of my true grt grandfather's family (council house workers and tenants) I was very doubtful that any one would have bothered sending their DNA. How wrong I was!
It's down to the pool of samples Ancestry have - which is getting bigger all the time. And when new samples match, they ping you - so it's on-going. Check out what I said earlier about using the Ancestry sample on other databases.