“It was completely inadvertent,” Daschle said, “but that’s no excuse.”He means getting caught...come on, these guys use a tax consultant/accountant to do their taxes. If he didn't claim it, it's because he was hiding it.
It's also likely that the reason he backed out fairly quickly is that he didn't want any more background investigation because there is still more dirt not yet uncovered.
Obama got off so easily during the campaign with a press that refused to investigate the large gaps and missing documents in his own background that he thought he could slide all of these corrupt appointees into office while the press continued to genuflect in awe at his very presence.