Filtering pointcloud with voxel grid giving different number of data points

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Hi, I try using the voxel grid filter on a pointcloud generated by the Ensenso camera. While the camera is held stationary, it gives a constant number of data points, however with a voxel grid filter of 0.01f (1cm). I've been received different number of data points back. Why is this the case.

      std::cerr << "PointCloud before filtering: " << cloud_blob->width * cloud_blob->height << " data points." << std::endl;
      // Create the filtering object: downsample the dataset using a leaf size of 1cm
      pcl::VoxelGrid<pcl::pclpointcloud2> sor;
      sor.setInputCloud (cloud_blob);
      sor.setLeafSize (0.01f, 0.01f, 0.01f);
      sor.filter (cloud_filtered_blob);           //pcd saved in cloud_filtered_blob
      pcl::fromPCLPointCloud2 (cloud_filtered_blob, *cloud_filtered); 

      std::cerr << "PointCloud after filtering: " << cloud_filtered->width * cloud_filtered->height << " data points." << std::endl;