Creative accounting? (Or, How many states?)

Tax prof: "And that [deduction is] the same in all 42 states. And I'm counting the District of Columbia as a state."
[Students all look puzzled]
Student: "What about the other states?"
Prof: "They don't have income tax."

(And I was thinking that either, (1) the professor was showing us the "unofficial" addition method to use when figuring taxable income, or (2) he had forgotten the addition of a few states since he was younger.)