The performance for your investment accounts is a calculation based on the account balance and cash flow (money in or out of the account) in the account. For any given date range, the money transferred into the account is excluded from the balance and money transferred out of the account is included in the balance. This ensures that any balance change is a result of the changes in your holdings.
Different financial institutions use different methodologies to arrive at this number and there could be differences in the performance numbers shown.
Note that the performance numbers in Personal Capital depend on successful aggregation of the account and on the accuracy of the data (transaction and balance changes) reported.