Hills and Guinness!
I think it's changed over time. Years ago everybody let on - perhaps there were fewer of us and we had to stick together? Nowadays, fell runners (and walkers) always nod and with road runners it is on a sliding scale from North to South. Up here most people nod. Occasionally, I find myself running in a large London park (must stop these binge drinking sessions!) and people either blank me or look astounded if I let on.
In my experience it's whether you live in a city or not. If you say hello to someone in a city it's akin to that scene in Crocodile Dundee where he's introducing himself to people on a busy sidewalk in New York.
People who live in a more rural situation are generally friendlier to strangers as there is a higher chance that they'll ever see them again.
The confusion comes when city types go into the countryside and aren't used to the friendliness of others. Or when country types go into cities and are mistaken for the mentally ill because they say hello to people they don't know.
'The birds are the keepers of our secrets'