Understood, but the point being that the only reliable and always-correct number to give to people is the amount withheld by the lottery.
You'd be surprised how many people incorrectly say that the amounts on the Jackpot Analysis pages at USA Mega are wrong. They say that the real amount is something like 39%, but they are not correct. The amounts listed on those pages -- 25% Federal, plus whatever the states withhold -- are the correct numbers, and then naturally people have to pay more or less than that on April 15, depending on what they did with the prize money.
Again, the point is that when people say the taxes on lottery prizes amount to 39%, that is wrong. For some people it may be 39%, but making a blanket statement that it is 39% is wrong.