IBP : KF Function Aggregation last period

Hello,

in the description of the function "aggregation last period" is a example of a disaggregation and aggregation.

in this example is a aggegation from 40 to 80 each. Can everyone explain me, where the 80 result from ?

Link to the description

https://help.sap.com/viewer/bf99e931b8d44aafb4e306ec3602cbdd/2002/en-US/8280eea63c914d8e95eecd322e5ab881.html?q=Aggregation%20last%20period