I usually get acknowledged when I'm on a road bike and dressed for cycling, less frequently on the mtb - particularly if I'm not in lycra, and almost never when riding the Brompton in civvies.
They're probably out of breath trying to drop you when you're on your Brompton. It hurts their pride when someone on a Brompton holds pace with them on their Colnago.
I often have to concentrate when riding in a group. You can't always trust the people you're riding with. But I generally nod to people I meet, even old grannies on their shoppers with baskets. They're no different to me, I cycle to the shops too.
I think it has more to do with the modern way of life, where we don't even talk to our neighbours anymore. It's seen as weird to greet a stranger and would genuinely get some people worried. I only do it on my bike, because I used to get other cyclists give me the nod when I was getting into cycling, aged about 12 on a sports bike from Halfords riding the back lanes of Hertfordshire in non cycling clothes.
I like cycling snobs though. They get very upset when a scruffy oik on a dirty steel fixed wheeler, carrying a saddlebag gives them a hard time on a hill.