Matchtemplate(Normalized Cross Correlation) same size two picture

edit

If src1 and src2 are same size. I know 'result' is one pixel.

I want to know why this method has this result. I want to know what value the second line returns.

cv::matchTemplate(image1, image2, corr, cv::TM_CCORR_NORMED);
return corr.at<float>(0,0);  // corr only has one pixel