If the base fare was $432 - $50 fuel surcharge, then you'd have a fare of $382. Since that's less than $400, they probably reduced it by only $82, to $300, then added the fuel surcharge to get to $350, and then taxes, fees, yadda, yadda, yadda to get to the $456.84.

Not to say that's fair or proper, but that's probably how they did it.