Part of it may be expectations - if it is assumed that people will be on time, a host of other assumptions follow: that a brief meeting is worth the time it takes out of one's schedule, that one should reserve a table immediately upon reaching the restaurant, that one will make the bus/train/flight to one's destination, etc. When they deviate from the cultural default, people need to be specific (maybe "The movie start time has tolerances (http://en.wikipedia.org/wiki/Engineering_tolerance) of +10/-25 minutes", or even just "Show up at least ten minutes early to the photo shoot").
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