It's not an observation I've had, but then I haven't been looking out for it either. If it is the case, I'm sure it's to do with the different employment practices between the USA and most European countries - minimum wage, security of employment, different tipping environment etc. Also, if you are talking about high end places, where waiting on table is more a career than a casual job, it could also be to do with preparedness to work late into the evening and get home after midnight on public transport.