If you open the account for edit and switch to Extended tab, you'll find a setting called "Update balance on each refresh".
If this setting is ON, then MoneyWiz will read the balance as provided by your bank and modify the opening balance of the account to make sure that the end balance matches that provided by your bank.
If this setting is OFF, then MoneyWiz will only read the bank's balance during the initial import, and will calculate it based on the new transactions. That may lead to balance discrepancies, which could be an indicator of missing transactions.
By default, this setting is set to enabled for new customers.