Because it could mean she didn’t tell the truth.
If your spouse asks you to take pictures of her posing with guns, would you easily forget major details about that event?
She didn't give a crap about guns, in fact called them something like 'man toys' She was always putting Oswald down, such as his musing about becoming an ambassador, or something. She laughed at this macho back-yard boy thing as well.
As for any inconsistency in testifying generally, I recall something about her referring to the fact that he was her husband; that the first thing one does is protect one's spouse.