The Frequency field is used to set up the Recurring Payment, which then holds the information on of the payment frequency.

If a value other than "One-Off" is selected when creating a recurring payment, the first Payment Txn will be set to that frequency, and then it create a matching Recurring Payment. Therefore all Txns created after that are set to "One-Off" in order to not create a new Recurring Payment record every time there is a charge.

There is a recently included field called Pay Frequency for the Payment Txn object, which has the frequency of the related recurring payment so the user can see this information on the Payment Txn. Add to your page layout if it's not there already.