I would argue that lying during an interview is an active lie (especially when they would have now lied twice, once on the form and now verbally), while not reporting something is more of a passive lie. People are more likely to do things passively… So if they aren’t questioned they don’t feel like they are lying.
Also, regarding catching spies and clearances - There is a lot of damage that can be caused by those who are not actually spies. People with loose lips (especially after a few drinks etc) or lots of debt (especially delinquent debt) can be susceptible to blackmail… If a person is deep in debt and someone offers them 50K to leave a door unlocked - we may have a problem. The person who takes up the offer may not be actively spying but they sure are a problem for national security. So keep in mind an adjudicator is also trying to evaluate how susceptible a person may be, given the individual circumstances.