The value shown on PeakD wallet page is fetched directly by the API nodes using this API call: https://developers.hive.io/apidefinitions/#condenser_api.get_dynamic_global_properties
As you can see there is a hbd_interest_rate field that give the current value.
But you are right, the value is dynamic and may change. As far as I know according to the followings:
- if top 20 witnesses change
- if one of the top 20 witnesses update his parameters
- according to what value is defined by the 21st backup witness
Thanks!