What is the difference between List transactions and List updated transactions ?
The API can delete / update / recreate transactions from day to day.
For example, if an account has a pending transaction its date can be updated when the transaction is debited from its account.
The List transactions endpoint gives you a snapshot of the user's account at the time you call the API whereas the List updated transactions endpoint gives you all the transactions that were created / updated / deleted since the timestamp you pass to the function.
This means that the transactions returned by List transactions have the is_deleted flag set to false.
List updated transactions should be used when your application stores a copy of the user's transactions on your server and you want to get an update from time to time using the since timestamp.