Average MPG calculation

Does anybody know how does The Prius computer calculate gas consumption or average mpg ? Today I was driving on a descending road with my foot lightly on gas pedal , while the mpg bar was showing 100 mpg , the mpg number below was barely 42.5 . I know that this number is calculated by taking previous readings , but how ? I have noticed while I kept an eye on the mpg number that the computer refresh it's reading every about 10 second .