Why does the flann based descriptor matcher match to different keypoints everytime?

Question

Is it normal? When I tried the brute force matcher, the result is consistent everytime, but flann is not. A small amount of keypoints will match to different places. I am writing the code using the Android wrapper, the keypoint detector and descriptor is SURF, something like this:

Mat queryDescriptors = new Mat();
Mat trainDescriptors = new Mat();
DescriptorExtractor surfDE = DescriptorExtractor.create(DescriptorExtractor.SURF);
surfDE.compute(queryImage, queryKeyPoints, queryDescriptors);
surfDE.compute(trainImage, trainKeyPoints, trainDescriptors);
DescriptorMatcher dm = DescriptorMatcher.create(DescriptorMatcher.FLANNBASED);
List<DMatch> matches = new ArrayList<DMatch>();
dm.match(queryDescriptors, trainDescriptors, matches);

Answer 1

According to Andrey, this is the reason! So yes, it is normal. To find more one would have to dissect the algorithms!

Answer 2

FLANN stands for Fast Library for Approximate Nearest Neighbors. The approximate nearest neighbor algorithms are non-deterministic, often randomized KD-trees.