The solution to the mystery? Personal distance. Personal distance, or interpersonal space, is what anthropologists and sociologists might define as the distance consistently separating members of non-contact species. But people, unlike animals, determine personal distance culturally, not genetically, and so acceptable distance varies widely from country to country.
An American such as myself in a queue at the bank to change money will stand directly behind the person in front, at a culturally acceptable distance of between 1.5-4 feet (0.5-1.3 meters). I would also be at pains to avoid looking at the transaction taking place ahead, lest I be thought nosy, or worse, a thief.
A Russian entering the scene will assume that I am waiting for somebody or am undecided about which line to get in, and will unashamedly sidle in to the right of what seems to be the only person in line, at a distance of about 1-2 inches.
If I don't immediately follow his example, others will come and do so, and I'll be standing, slightly to one side, of a line of a dozen people, still not getting my money changed. But personal distance is not just about proximity. It can be about emotions too.
The Russian "neutral" expression is a blank, unsmiling face, which appears forbiddingly angry to Americans. Americans in turn, often appear to be vulgarly laughing at strangers when they automatically smile at people on the subway. Eye contact is also varies between the cultures. Russians often seem to be staring rudely by Western standards of eye contact, which allows for little or no eye contact between strangers. When in 1993 I brought to Russian a group of American students, without exception they wove elaborate paranoid fantasies about harmless Russian strangers "staring at me with this evil expression."
Conversely, they all had a terrible time getting sales clerks to respond at counters when using the standard American method of simply staring at the clerk till she says "Can I help you?" Fact is, around here you can stare for a week and not get service until you politely say "Devooshka?" ("Girl?").
A problem for visiting Americans is that Russian personal distance lies within an American's intimate distance, just as an American's personal distance lies within northern Europeans' intimate space. The result is that Russians seem pushy or over-amorous to northern Europeans, and Europeans seem cold, and unfriendly to Russians. Americans, existing somewhere in the middle, manage to equally offend both parties, for opposite reasons. And this affects sexual, as well as other relationships, between the nationalities as well.
To American men, all Russian women appear to be flirting outrageously. It is that level of unintentional flattery that makes Russian women so sexy to western men. On the other hand, many Western men, by using their own national norms for greater distance, little touching, and low eye contact, strike many Russian women as far more "respectful" and "polite" than Russian men. This, as much or more than economic factors, contributes to the growing trend for US-Russian romance. The key here is awareness. Westerners living in Russia need to temporarily adapt their spatial relationships to the Russian style for day to day survival.
Conversely, Russians working regularly with short term tourists (who won't have time to adapt themselves) need to respect the spatial conventions of the nationalities they service, or risk annoying their customers. Once you understand the rules on space, it's easy to fit in.