Why SIFT and SURF detects keypoint in white circle?

Question

I'am playing with the SIFT and SURF algorithm. I'm trying to figure out why does the SIFT and SURF detects keypoint in the center of the circle shown in the image below. Any ideas please? The first photo is corner Harris detection, second SIFT and third SURF.

Answer 1

SIFT (Distinctive Image Features from Scale-Invariant Keypoints) detects

stable keypoint locations using scale-space extrema in the difference-of-Gaussian function.

From what I have understood, a blob in the current scale can be seen as a small dot in another scale space.

It should be the same thing with SURF (SURF: Speeded Up Robust Features).