PW2 and time left in chapter

How does a kindle ereader calculate "Time left in chapter"? For me, the stated time is always wrong. It always gives a time which is too short. I've timed this several times and found it takes me far longer to read a chapter than the kindle has estimated.

Does it use an average?
Does it include the times I rifle through a book, looking ahead for information?
How does it cope with instances like putting the kindle aside to "powder my nose"?

I'm sure that the answer is very complex; I'm mathematically challenged, so please keep answers as simple as possible. :thanks: