You may see a 0% unsubscribe rate or action rate from time to time because these fields represent a percentage of the total recipients. That percentage is rounded using the prime rounding system to the nearest whole number, so if your unsubscribe or action rate is less than 0.5% it will round down to 0. If you require more precise rounding I would recommend filing a feature request via your "Ideas" tab, or contacting your CSM about a customization.