Pinhole Acuity Test

I have a question about an anomaly I experience when using an optometric grade pinhole paddle. When I view a Snellen chart through one of the pin holes I see a cloudy area that occupies part of the center of the pinhole view. The cloudy area only obscures about 50% of the view through the pinhole. However, I am not able to see past the cloudy center. I can see the chart on the periphery of the cloudy area up to the outer edge of the pinhole. Does anyone know why the view through the pinhole is obstructed by this central gray area? I am doing this without glasses.