If you open a Lincoln welder like the one you bought, you will see that the different amperage output settings are achieved by tapping different locations on the transformer with individual conductors which run to separate connection points on the back of the selector switch. Part of the reason that this welder has only a 20% duty cycle at every setting (except for the 75 amp setting which is 100%), is that the conductors are not all the same size. They run from the smallest conductors at the lowest settings to the largest at the highest. Since the duty cycle is a function of the amount of heat which is generated during use, using conductors which all generate approximately the same amount of heat per unit of time creates a situation where lower amperages do not result in a corresponding increase in duty cycle.
The situation is actually somewhat more complicated than I have described (a more complete explanation can be found in the archives of this site), but it is reasonably accurate---at least in its result. Lincoln 225 amp AC and AC/DC welders are famous for robustness. Many, many people report using them without regard to the duty cycle with no problems. While I wouldn’t go so far as to recommend that approach, I think you’ll find that the 20% duty cycle won’t pose a limitation for regular usage.