See this:
http://www.bikeforums.net/19526349-post18.html
See the welding section perpendicular to the hinge face? That is MUCH stronger because the failure mode is shear stress whereas with the butt weld the stress is normal, nowhere near as strong. The longitudinal weld is on an internal web also shown through a hole in that post. The best case would be if that internal web was one piece with the hinge plate; in this instance it is also welded to the hinge face, so effectively all the hinge face welds add together for total weld strength. The extra welding over and above the original welding is therefore effectively the extra bit that attaches the internal web to the hinge face.