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OpenCV Distance Metrics

Hello everyone, I'm doing a benchmark test on keypoint detectors, descriptors and matching of those for a research project. I'm aware that there are great benchmark tests out there but this will be done for a specific experimental environment. Descriptors will be Orb and Freak and detectors are ORB, SURF and maybe FAST. I will use BruteForceMathcer.

So, in order to make the test fair, I decided I will use approximately same amount of keypoints and the same distances. But I can't seem to find a detailed explanation of the distance metrics used in opencv.

For instance; when I use "BruteForceMatcher<l2<float> > matcher;" when both keypoint detector and descriptor is orb, it gives me a correct match between two points with coordinates point1(130,339) and point2(130,340). Obviously the distance between those two is 1, but when I look at the matches vector, the distance value is 272.71964 which is very confusing for me.

My question is, is there any documentation that explains why this is the case? I googled it, but haven't found a decent explaination. If not, I would really appreciate if you could explain this.

Thank you