It's fairly straightforward to see how revenue vs. old Site is calculated - it's the comparison of revenue from the experiments on new layouts against revenue from the old layout. Calculating performance vs old site is not that different - it still uses comparisons of metrics from new layouts against the old, but this time it uses a blend of revenue, time on site, page views per visitor and bounce rate. Depending on whether your optimization settings are Revenue Focused, User Experience Focused, Balanced or Custom, each metric will be weighted to ensure the system promotes layouts in accordance with your desires and goals for the site.  

Some metrics are much more difficult to influence than others, so standard deviation calculations are made to ensure the movement of the metric influences the performance results in a proportionate manner.