Why the threshold for RANSAC set to 20?

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AirSLAM/src/point_matcher.cc

Line 97 in 2b82516

cv::findFundamentalMat(points0, points1, cv::FM_RANSAC, 20, 0.99, inliers);

I wonder why the threshold for RANSAC set to 20? It is so big for standard SLAM application. I try to set it to 1.0 leading to a worse accuracy.