Average time spent on site for person ID dimension does not tally with the average time spent on site for form ID dimension

[Serene2]

Hi, i need some help. Can someone help to explain why the average time spent on site for person ID dimension (i.e. 36 seconds) does not tally with the average time spent on site for form ID dimension (40 seconds)? These individuals under the person ID have visited form 1. 

Form ID dimension	Average time spent on site (seconds)
1. Form 1	40
	Person ID dimension (total 1,553 records)	36
1. Person A	362
2. Person B	332
3. Person C	334
. . .	. . .

[Jennifer_Dungan]

It looks to me like there may be instances of "Form 1" that don't have a Person ID dimension.. so the average is calculating "Person A, Person B, Person C.... and Unspecified"..... but in the breakdown, you are only showing Person A, Person B, Person C...  (unspecified isn't being accounted for), hence the slight difference in the average