I have noticed a clear division between what bike you ride. I'm on a Honda Hornet and I would say......90% of sportsbike riders nod back, where as only probably 50% of cruisers nod back. Maybe if I was on a different bike that would be turned around? Almost like bike snobbery.
I've seen that here in the States, although that seems to be diminishing where I live. Back in the early 1990s when I started to ride, The Harley guys wouldn't wave or would even talk shit to someone riding a "rice burner", meaning especially sport bikes, but even cruisers if they were Japanese and even Ducatis, which are obviously not Japanese.
Sport bike riders mostly waved at sport bikes because they knew Harley guys didn't like them.
I rode a Harley back then but I learned on a Kawasaki 440ltd, borrowed from a friend who really knew how to ride, so I pretty much knew all that was bullshit.
Nowadays, I think there are so many new riders, a lot of them in their 40s and even 50s, riding so many different bikes that those old attitudes are going the way of the Dodo. Most of the guys who are old enough to remember why we hated the Japanese in the first place are getting too old to ride. Harleys are easy to get now, whereas in the 1990s, you had to be on a waiting list to get a specific model and color of HD.
These days I wave and get waved back by just about everyone, including guys in MCs who used to not wave at anyone. If you watch my vid "Meeting Cool Riders in Dallas", I exchange pleasantries with a guy on a HOG and a guy on a Suzuki. For me that's typical.