This site may interest you too...
www.statbrain.com
For my 30k site it guesses 45,651
For my 10k site it guesses 7,907
For my 7k site it guesses 137 (lol)
For my 5k site it guesses 352
If you were able to get real data for a large number of sites and compare them with estimated data you might find that when averaged they're sort of right... but at the individual site level you'll see a lot of these significant fluctuations; the estimates are dead WRONG.