average time spend on site - is depending on which dimension you are usinga and counts the total time divided by the sequences
time spent per visit= time spent/visits
Visit --> page a, page b, page a, page a --> all pages 1 second
average time spent on site for a= 3 seconds divided by 2 sequences = 1,5 seconds
time spent per visit for a = 3 secondes divided by 1 visit = 3 seconds.
This is how it gets calculated