As the other comment pointed out, they have a set of very particular facts and will press you with questions about dates, times, amounts, who, what, where, and when. If you say a not true statement about one of those things they charge you with lying. The relevant law is 18 U.S. CODE § 1001. It's 3 pronged and the 2nd says "makes any materially false, fictitious, or fraudulent statement or representation; or". I don't believe there is a requirement in this law for criminal intent, caveat IANAL.

Suspects often trip themselves up lying to federal law enforcement officers about stuff that's not even really sinister or criminal. They tell small lies to avoid embarrassment or make themselves look better. People tell little white lies all the time in regular conversations and find it hard to break the habit when being interrogated by the FBI. Then when they get caught in a minor discrepancy they double down with bigger lies and dig themselves into an even deeper hole.