They just iterate over every dictionary word and bolt on numbers and extra letters, or you'll see stuff like genewitci@gmail.com henewitch@gmail.com, etc. I haven't paid attention to gmail in a couple of years since i now use my own domain and fastmail (for $5 a year, even), so i have no idea if the mailing lists are more refined now or not.

Nearly everyone on earth knows about the dot separators and the +whatever that gmail allows, and will just trim that before they sell your email address, nullifying the usefulness. having someuniqueID@example.com is much nicer. you cut down on spam a lot, however it does open you up to a lot of spam to admin@ and webmaster@.