matchFeatures does not match all features when comparing the same image. Why?

Hi. I have an image I (greyscale). I detected and extracted SURF features from this image :

poi = detectSURFFeatures(I);

[features,validPoi] = extractFeatures(I,poi,'SURFSize',128);

373 features have been found. However, when I count the number of features matching to themselves, not all features match:

indexPairs = matchFeatures(features,features);

index pairs returns 365 matches only. Why ??? Note that I found this results for one image out of three only. For the other two images, matchFeatures returns the same number of features as extractFeatures. Thanks !