getBudget should be documented as returning the amount of budget that there is expected to be assuming the user doesn't interact with the site at all in the meantime (i.e. a weak lower bound, though it's possible the budget will go below the prediction if e.g. the user revokes a permission for the website).
Otherwise UAs can't make reasonable predictions, and getBudget would be less actionable for developers.
getBudget should be documented as returning the amount of budget that there is expected to be assuming the user doesn't interact with the site at all in the meantime (i.e. a weak lower bound, though it's possible the budget will go below the prediction if e.g. the user revokes a permission for the website).
Otherwise UAs can't make reasonable predictions, and getBudget would be less actionable for developers.