That's b/c you data sets don't match up. If you perform a simple 3D distance check between each pair of points, you'll find that your first two points are 1419.4mm distant in both sets. But your distance between P2 and P3 in the robot is 971mm, while the P2-P3 distance in Base is 231mm. The P1-P3 distance is 1661 vs 1371. So you're trying to fit two triangles that only have one side of equal length. For the fit algorithm to work, both triangles must be nearly perfect matches in size and shape -- the algorithm only calculates the positional and rotational differences between the two triangles.