The groceries thing is funny, but it's really just "what's the likelihood that in X years of living in the same place, your random 5 minutes out of the week are the same as your neighbor's random 5 minutes of the week".
It means that you can see your neighbors do groceries almost every week if your schedules match, or never if they don't.
I noticed that with people I met in the elevator. Some people I met almost every other day, others, never. Since the beginning of the pandemic, things changed, with remote work and constantly changing policies and restrictions, things became much more random, and I meet more people, but less often.