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OpenCV Why does number of SIFT keypoints go over threshold?

Hi,

I created SIFT constructor, with Threshold = 100

cv::Ptr<feature2d> f2d = xfeatures2d::SIFT::create(100);

f2d->compute(image, keypoint, descriptor);

Then I extract descriptors from sequence of images and for some images I receive number of keypoints bigger than limit by one. Please see descriptors print below:

[128 x 101]

[128 x 100]

[128 x 100]

[128 x 101]

[128 x 100]

[128 x 100]

[128 x 101]

[128 x 100]

Why does it happen? Do have incorrect understanding of nfeatures attribute?