The birthday "paradox" explains this exactly. The issue isn't Gray Close. It's not three of something, its about three of anything. Imagine that there are 2717 days in a year and there are 11 people in the room, then the odds that two of them have the same birthday are about 2.4%.

If you break Newham into household zones each containing 34 households, then in any 11 calls, you would expect two to be in the same zone every 41-42 days, assuming you never got a day off.

What is the math for three people in a room having the same birthday? I have no idea, though I suppose if I weren't so addled with 1975 Bordeaux, I could work this out. Obviously, less than 2.4%, but probably not amazingly so. I'll bet that you 3 out of 11 in the same "zone" every few years. Draw your grid now.

Of course, there are an awful lot of undistinguished 34 household zones to consider.