This is an example of discussion which I have posted on this board many times.  Review turn around time has to do with how many reviews that are in the queing awaiting review and approval (although I have never heard of waits longer than a week).  The short waits (typically less than a day) are during slack periods such as around holidays and during summer or christmas vacation seasons when the ques are less.  It is sort of like waiting your turn "on hold" on the phone.  Real bitch but characteristic of the realities of life: never enough people to do the job.