I was comparing hot trend graphs of the same term for subsequent days.
I knew that the search volume (ordinate) might have a different scale.
I knew that the time zone is PST.

I thought I would be able to put two subsequent days side by side.
I was surprised that subsequent days used different starting times.
Below is an example comparing the term "John McCain" on January 3, 2008 to January 4, 2008. Look carefully at the times.
The January 3 times are from midnight to midnight.
The January 4 times are from noon January 3 to noon January 4.
The same peaks can be seen in both graphs.
Similar behavior seems to also occur with other terms and other dates.

"John McCain" January 3, 2008
http://img504.imageshack.us/img504/8242/mccain080103zt0.png
http://www.google.com/trends/hottrends?q=John+McCain&date=2008-1-3&sa=X

"John McCain" January 4, 2008
http://img72.imageshack.us/img72/8863/mccain080104iu8.png
http://www.google.com/trends/hottrends?q=John+McCain&date=2008-1-4&sa=X

I see nothing abnormal here.