图像均值和峰度-Matlab

Question

为什么使用矩阵或直方图的图像均值和峰度结果存在差异？正确的方法是什么？

代码：

rgbImg = imread('peppers.png');
grayImg = rgb2gray(rgbImg);
imgHisto=imhist(grayImg);
meanGrayImg = mean(grayImg(:));
meanImgHisto = mean(imgHisto);
kurtosisGrayImg = kurtosis(grayImg(:));
kurtosisImgHisto = kurtosis(imgHisto);