Re: WHY DIFFERENT
Costs for a click on an ad change frequently as the circumstances of the auction change.
Factors that can affect click cost include: the precise search query the searcher used, time of day, day of week, level of available traffic, some competitor ads being active/inactive at the time, and many more factors.
In this specific case, I would say that day of the week (Friday versus Saturday) may have been a major factor.
Hope that helps!
Google AdWords Top Contributor