If a statistic of a mask is computed, also the statistics of the complementary mask are computed. See
template <typename TPixel, unsigned int VImageDimension> void ImageStatisticsCalculator::InternalCalculateStatisticsMasked(typename itk::Image<TPixel, VImageDimension> *image, const TimeGeometry *timeGeometry, unsigned int timeStep)
[...] std::list<int> labels = imageStatisticsFilter->GetRelevantLabels(); auto it = labels.begin(); while (it != labels.end()) { [...]