portfolio rate of return account, question, why, already

Roflnadal

In note 5, page 110, question 1
The account was valued at $6000000 at time0, $6380000 at time1,  why when contribution was added at the begining, it has to be added to $6000000 (so it does not contain the contribution, so need to be taken account), but why when the contribution was added at the end, it has to be removed from $6380000 (why it already contain the contribution, seems not consistent)??