She also told the judge that she put a bottle of water in the box for the dog. I’m guessing that came up in the mental competency hearing - Along with her putting little holes in the box for air, but which she sealed over with tape when she was sealing the package.
Which does not explain her lie. She would only lie about a live animal being a robot if she knew that she could not mail a live animal in that manner.
People lie when they believe they have more to gain from lying than telling the truth.
If she really believed it was a robot, she would never have put a battle of water next to it. Robots don’t need water bottles.
If she knew it was a dog, but didn’t want to pay the necessary cost of shipping a live animal to the buyer, she has a motivation for lying.