It's not unheard of for a bearing set to have one odd-sized bearing half. Check the markings. If one is marked wrong, you know the problem.

Otherwise, as sixto implies, you might have a bad bearing half. Plastigauge won't necessarily show a bad half or even one of the wrong size, as the binding won't be evenly distributed. Definitely try another set in this rod to see what it does. Then you'll know.