Considering the amount of the ticket, I'm guessing this is international travel. The difference in your fares is $162 which could very easily be a tax issue.

Are you looking at the IATA fares directionally?

Just because its $4530 between SEA and XXX doesn't mean that its $4530 from XXX to SEA. Certain taxes are included in the base fare and are charged based on departure country. That could very easily make up for the $162 discrepancy in the calculations you've given.