The little extra amperage you can use for derating purposes like for wire count in conduits and ambient temperture corrections.
Welders wired under provisions of Article 630 Cord can use derate wire sized based on its duty cycle. The thought is with a 60% duty cycle meaning it is not to ran no more the 36 minutes per hour(60% of an hour). Even if the welder is cranking out 100 amps which will be rare with you welder if properly sized for the work, the cord will not get hot enough to cause damage. Besides the cord is part of the equipment which is not covered by the NEC. The outlet though is a different story. In theory, some other piece of equipment could be pluged in the the receptacle. The circuit must be able to carry the load.
The welder should be wired per the maufacuter's instuction. Failure to do so can void warranty and listing. If you void the listing, you can not use it. NEC requires all electrical components be listed or approved by a third party tester facility such as UL. That is why you see the little stamp like on a 4" square blank cover. Although it is just a square piece of metal, it still must be listed.
