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Hi Yianni,

if the rotational invariance of the features is used there can be more than one descriptor per keypoint, if there was more than one strong rotation around the normal found for the extracted range value patch. Additionally, the number of descriptors can be lower than the number of keypoints. E.g., if the resolution in the area was too low.
The values of the descriptor are normalized to -0.5 and 0.5, so that the difference between descriptor cells is between 0 and 1. I typically use the Manhattan distance divided by the descriptor size as the descriptor distance, which will always be in [0,1]. I prefer my values to be in areas where an interpretation is easy, which is why I use this normalization.

Cheers, Bastian