What you mentioned at the top basically explains the difference between serving Americans/Canadians and Russians. I’ll spend an entire night chatting with Americans which some have told me is more of a thing in Canada but I think it depends on the state. I chatted with a couple from (I think) Michigan today for a while after they had already paid and I could have finished my shift. The other week I served a group of Russian guys and they were friendly but definitely more about their “tribe” like you mentioned. They were never rude but I don’t see them striking up unnecessary conversation either. I brought them a new pitcher of water before the other ran out and got a surprised smile for that though which I have yet to understand. Some people would probably consider that rude I guess. When I was a cashier we were suppose to make conversation with guests because it is considered rude/awkward not too.
You helped Russians when you didn't have to: they didn't order or asked for a new pitcher, and you brought it anyway. The smile you'd got was an indication that your help was perceived pleasantly. For Russians, that's entirely unexpected. The service industry in the US (and, seemingly, equally in Canada) is aimed at pleasing the customers. In Russia (and post-Soviet territory, in general), it's aimed at delivering your order; the pleasing part is currently being introduced, slowly and with a local accent.