It's a completely expected result with @api.one

@api.one is decorator, that may used instead of @api.multi, if you want to iterate automatically over "ids" (that is recordset in v8.0+) passed to the function. If called function returns anything, then @api.one collects results from each call of function into the list, and finally that list is returned. 

So, if you'r function do not return anything, then you're free to use @api.one without difficulties, but if your function return something, then you'll need to consider the above described behavior and if the resulting list does not fit into your requirements, then fall back to the @api.multi and use it. if you want to make sure and check additionally that "self" contains exactly one record, then you can check it with ensure_one() function:

    @api.multi
    def _compute_stop_time_hrs(self, start, duration):
        self.ensure_one() #One record expected, raise error if self is an unexpected recordset
        return self.start_datetime