Fridge Logic: In Jim's first appearance, he struggles to remember his last name when filling out a form. He's asked: "What was your father's name?" To which he replies: "Ignatowski." However, Jim's family makes an appearance in a much later episode and his father's last name is Caldwell, and that Jim changed his name to Ignatowski because he thought it was "Starchild" spelled backward.
The obvious explanation: Jim's brain-circuitry is so fried, he could have said anything.