From the USPTO website, I found this ex parte re-exam statistical information from July 1981 to Dec 2010:

7. Overall reexamination pendency (Filing date to certificate issue date)

a. Average pendency 25.6 (mos.)

b. Median pendency 20.0 (mos.)

Seems obvious to me there's an average time of 25.6 months between the re-exam filing date and the completion date. The average stat means the average time for a single re-exam. Doesn't median refer to the number (in this case ...months) that occurs in the middle of a set numbers (the set in this case being the number of months of all the patent re-exams since July 1981)? If so, I don't understand why the median number is 5.6 months less. Anyone have an idea? The median stat could be used here due to re-exam cases being filed but are still pending, so the end-values are unknown, and those re-exams are removed from the dataset.

I have an email into the USPTO for clarification.