The grid system was inherited from print design layouts and follows the same concept as the photographic rule of thirds . The idea is to align the content to certain boundaries to make your layout receptive โeasyโ to view and understand.
960px comes out of an attempt to set within the lowest common denominator (possibly 1024x768), and 12/16 column layouts are probably the highest practical dividers.
I do not think that you will find a specific technical answer for this, since it is rather a subjective aesthetic technique. Someone with the best design / linguistic abilities could describe it better :)
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