Isn't it like lottery numbers? Like, if you had 40 numbers, the likelyhood of getting any one is 1 in 40, and it increases slightly for each number you draw out, and kind of 'resets' each time, so that the likelyhood of getting 1,2,3,4,5,6 is the same as getting any number. In the same way, you're just as likely to get three houses in Gray Close as you are of getting any three houses out of the same size sample. It's one of those weird things where if you went to three houses in Gray Close, that's as likely as a house in White Ave, Black Street, and Orange Lane. However, if you went out before your shift and chose which houses you'd get call outs (which, strictly speaking, should be 'calls out', given that to call is the verb here, and you get 'called out') to, then got them, that'd be highly unlikely (like picking winning lotto numbers). But then again, you do get regulars, so I guess that warps the sample to some extent. That probably doesn't make sense, I'm much better at crosswords...