I highly doubt that would be a factor in that decision.
1920x1080 is native resolution only for iMac 21.5-inch and Mac Book Pro 17-inch(1920 by 1200) so it probably isn't the most widely used resolution among apple users.
And most of 960 grid layouts actually use 940px for content (20 pixels are left for margins) - so that would fit with scrollbars anyway...
Also, even if you would have two sites using full 960px, you would still have to align windows with pixel perfect precision not to cut anything out.
