The replacement chassis will of course have been welded together by the same people as the one you want to replace...Catch 22 anyone?
I will absolutely bow to Knucklefreak's knowledge but will take a guess that the welding is flux core (MIG) a process often carried out by "semi-skilled welders". Set up and prepared just right it works beautifully and quickly, not quite right or dubious weld prep and like all weld processes it can be pretty damn poor. You have to hope that when it isn't working just right the welders have the knowledge and ability to recognise what is happening and can adjust things back into the zone.
