You are likely diluting the spirit with the water wetting the carbon being displaced, especially if you are running in batches. The procedure I have heard is to run "storage" alcohol through the filter tube after wetting, and measureing the output until it reaches your input proof, meaning all the water has been displaced by alcohol. Once you proof is back up, separate that first collection for storing the filter after you complete the run, then proceed with your standard product filtration. I am still new to carbon myself, but this seems to be the likely culprit; hopefully others more knowledgable will hopefully chime it...