As a general observation, the end user doesn't know the physical layout of the chart.
Specifically, where discontinuities exist and the extent of non-linearities.
While one can get around some of these problems by crossplotting that is beyond the line pilot's approach to the question during operations.
Therefore, unless the operations manual provides specific guidance regarding interpolation techniques, it is better to take the published conservative data point and accept whatever penalty is associated.
Re the specific question, and acknowledging nil specific background with Airbus, why ever would one use other than the nil wind line if the wind is calm ? There might be some rationale in L&V conditions.