The Online Activity page contains all transactions initiated via online banking, but does not include transactions made via other means, like ATM or Debit/Credit card transactions. This page also shows the status of each electronic transaction as it moves from a drafted status to approved, then processed.
In contrast, the Account>History page is a record of processed and cleared transactions against your account from all sources, not just online banking. Additionally, account history does not include transactions that have been drafted/approved/cancelled via online banking, but only those that have already cleared or will clear your account.